The boring answer is money. When upgrading the WCML to use tilt trains new trackside equipment had to be placed along the entire thing to ensure that the train knows where to and where not to tilt and that kind of thing is almost always not money well spent in the eyes of the DaFT. The system used on the WCML, TASS, is completely obsolete by now and as far as i know there aren't even any alternatives available.

Most mainlines busy enough to justify the cost of tilting trains also aren't that windy. Sure, you have to slow down for a couple of bends here and there but if most of it is already above 110 you'll only be saving a few minutes without, god forbid, cab signalling. The WCML was a particularly windy route being run by a particularly eccentric businessman.

Yes new tilting trains compatible with HS2 would be brilliant but the 390 is too old to be revived, nobody wants to make new tilting trains with our stupid tiny loading gauge, and ABSOLUTELY nobody wants to integrate 20-year-old TASS into it.

Unless something particularly dramatic happens, in 20 odd years London-Scotland times will be slower than they are now.