What is the attraction of a steel bike?
Know very little about them!
I think it's a number of factors:
Long ago in the pre-aluminium age the best bikes were made from 'fancy' steel tubes from Reynolds, Columbus, Vitus of which Reynolds 531 was the one most of us could afford and it made lovely (but not stiff) bikes to ride. Each was lovingly hand crafted and painted, lugwork could be plain or fancy etc. Aluminium came along and was lighter, stiffer and cheaper thanks to being mass-producable. So an element of the whole 'steel' thing is a certain nostalgia for bikes that we used to dream of.
Another element I've already alluded too is that steel frames are made by a man with a frame jig and a welding torch, an artisan if you like, hand-made. Paintjobs can vary greatly for the same frame, lugwork differs, and can be lined etc. OK, many frames were hand made on a production line, but many like say Pearsons in Sutton had a guy out the back making them any way he fancied, so hanging in the shop was a row of distinctly individual frames. You could have a custom job made almost anywhere.
Also, it was quite a cottage industry, some became more mass-market than others, but a lot of old frame-makers were very local concerns.
Finally, steel bikes all have a certain ride quality that is different to Aluminium, Titanium or Carbon. It's smooth, springy, mildly shock-absorbing, it's quiet too unlike Aluminium and Carbon which can be clattery over rough ground. You either like it or you don't, but it's very pleasant to ride.
Finally, steel bikes last for donkeys years. They can be endlessly restored, modified and repaired (for a cost) and can be a joy for ever!
Try one!