Space in the deep tube tunnels is really tight[0] (making it tricky to fit on-train air con radiators) and there's insufficient air circulation in the to clear the extra heat emissions in any case. They're working on both, but (unlike the shallower cut-and-cover lines and the NY Subway, both of which have saner tunnel sizes) it's decidedly non-trivial.

[0]: https://commons.m.wikimedia.org/wiki/File:Why_London_Undergr...