On why anyone would want a steel frame in these days of carbon fibre, titanium, and aluminum.
There are three good reasons why steel frames are not dead. 1) Design, 2) longevity and repairability, and 3) aesthetics. I shall discuss each point separately in more detail below.
But first, let me explode a myth. There are literally thousands of frame builders the world over who make first-rate frames from carbon, titanium, or aluminum. So why would anyone prefer steel? It's heavy, right? And who wants a heavy frame? Realistically now, we are talking about a pound (roughly 500 grammes) difference between a first-rate carbon frame and a steel frame of the same size and similar quality/price range. As David Miller once said, "it is easier to take a pound off the rider than off the bike." I readily grant that 500 gms might make a difference in time trialing--the difference between the victor and the also-rans. 500 gms might translate into a millisecond or two. But only for the ten or twenty world-class riders at the top of their form who have trained for years and done nothing but train for years and who are professionally coached and who have sponsors who pay for everything and whose inspirations, expirations, perspirations, and excrement are scientifically analyzed and and and. And that is not us, by and large. Those who could really coax a millisecond out of those 500 gms are a very select elite. For the rest of us, 500 gms off the bike is a poor excuse for not having ridden those 500 gms off our bellies instead.
Carbon frames have come to dominate the mass market because carbon frame manufacturers can't make enough profit selling their wares to a dozen world-class teams. It's pure marketing (profit mongering) which led them to make the man on the Clapham omnibus think he NEEDS a carbon frame. It's like selling Formula One racing cars to businessmen to go to work in. Well, maybe not quite, but it's not far off.
Now let us see what advantages steel offers over its rival materials.
First, design. If you are happy with an off-the-peg frame, if it suits your needs and fits your body, then there is no objection to buying a frame made of carbon, aluminum, titanium, or even bamboo for that matter. But as soon as you require any special design elements or concepts, you are in the realm of bespoke frame builders. Although there may be a few bespoke frame builders working in titanium, you will certainly not find any working in carbon or aluminum; but you will find many working in steel. There is a very good reason for this: the properties of steel lend themselves to this. Carbon frames are made in moulds; one cannot make a modification to the design without making a new mould--and no customer would pay the price of a new mould for a single exemplar (hence, the pressure on carbon-frame builders to go down-market to sell their wares and recover their development and production costs). No such limitation applies to steel-built frames. The steel-frame builder can braze together any combination of angles and fiddly bits as a one-off without incurring any extra costs.
Perhaps an off-the-peg frame suits you. Fine. You got lucky. Be happy. But what if your torso isn't proportioned with respect to your legs quite as the 'model' used for any off-the-peg frames you can find? There is no default human. An off-the-peg frame can fit pretty well, but for a perfect fit, for a bike you won't have to accommodate yourself to, go bespoke. And that means steel.
Perhaps an off-the-peg frame would fit you perfectly, but you might still have some very special requirements. Maybe you've got the use of only one hand, so you need a special arrangement for the brake lever which actuates both calipers and shift levers all on one side of the bike, and that requires some special braze-ons. Or maybe you want a short-wheelbase tandem which fits a much taller stoker than the captain. Or maybe you're planning a round-Africa trek with some colleagues and you've got special kit to take along which requires careful planning which bike is to carry which bits of kit, and you want Rohloff hubs and hydraulic brakes all round, and oh yes, one bike is to pull a Bob Yak trailer.... You get the idea. If you put your trust in off-the-peggers, you're going to face compromises and possible breakdowns later on. Steel is the material of choice here, hands down. Only a steel-frame builder can offer you the flexibility, at a price you can afford, to get both the total concept and all the details right. Granted, those are extreme cases. Nonetheless, even if you're not so far off the beaten track, you might still take a look at the following link, or better yet, read the book, to see what all goes into designing a bespoke bike:
It's All About The Bike, the pursuit of happiness on two wheels.
Second, longevity and repairability. Carbon and titanium frames have not been around long enough for us to know how long they will last. Steel and aluminum frames have been; they last upwards of a hundred years. Come back 80 years from now and see whether carbon and titanium frames have stood the test of time or whether most have failed by then. (Of course, the professional teams which race carbon frames don't need them to last a hundred years; THEY don't even need them to last to the end of the race--they have spares on the pursuit car. But that's not us, be honest; we don't go round with four spares on top of a pursuit car.)
Of all the materials used for bicycle frames, steel alone is repairable. Crash a carbon frame and you can throw it away; crack or bend a titanium or aluminum frame and that's the end of it; no one repairs them. But a steel frame can be repaired. Anywhere in the world. If a dropout or a fork crown should fail, even in the Hindu Kush, any blacksmith in the next village will understand what is wrong and weld on a tab to get you going again. It may not look pretty, but it's better than walking the rest of the way across Asia.
Third, aesthetics. Admit it: carbon is ugly. Carbon frames look like wash buckets. There are nicely anodized aluminum and nicely painted titanium frames. Klein made aluminum lovely to look at. Early Speedwells were lovely too, with their brushed bare-metal look and fillet brazed joints--too bad they broke (see point two above). But the clear winners in aesthetics are lugged steel frames, like marble compared to linoleum--no contest. Lugs are a splendid union of form and function, adding strength and beauty. Steel can be painted and re-painted any color you like, and steel alone will take chrome. So who cares about aesthetics, you ask? Evidently enough customers with taste to keep several hundred bespoke steel-frame builders world-wide busy even today.