Because Scotrail have far more important priorities than to cater for wishful nostalgic minority.
Scotrail's responsibility with the HSTs is to ensure it's fit for the purpose it acquired them for, and to utilise it as much as possible to represent a true intercity network for Scotland, with its own new identity. Why on earth should they give a damn about the IC livery?
Nobody suggested the GWR and GNER liveries were old because they weren't 100% replicated and reused 20 years after being replaced. The IC livery however, has.
I don't want to sound brutal here but for the sake of reality, it's time to live in it. A tribute livery is fair enough as a one-off for a limited time, but to make it permanent again just defeats the purpose of creating a brand new identity for the routes they are going to serve up here