Price has a lot to do with it for me. Old good quality rigid steel MTB's from the nineties make very good commuter bikes . With all the winter salting here they will not last very long so I will not spend a lot on the bikes I use during winter. A modern bike costing the same as the old one is heavier and much less fun to ride. One will have to be able to wrench for this to make sense though.
I will however stubbornly insist that my 87 Team Miyata is a really good bike for a 4 hour ride on bad tarmac regardless of age !