Here's 2 reasons I can think of...

Better productivity (no rest days) and Incentivises/retains staff (gives a benefit in a rather tax-efficient manner.)

That productivity point could easily be worth more than £1k if you save two billable days and 24hrs worth of expenses.

Problem is, the accountants get to work and deem the travel budget needs to be cut by 20% or whatnot. They do this by blocking J class travel, not looking at the fact that billable days fall or the fact that staff then become reluctant to travel.

I think we're just coming to the end of a protracted phase where employers could take the proverbial.

You probably can't justify spending £600 on a J ticket LON to FRA when the same can be had in Y for £100, but long haul is a different kettle of fish altogether.