Where would the waste heat go? Air Conditioning units dump the heat "outside" into the air. But there is no "outside" in deep tunnels. Worse, Air Con has to work to move heat, which creates more heat. You can't even break even. See the 2nd law of Thermodynamics.(1)
It's not so much "air con on trains" but "air con in deep tunnels without a vent". There is nowhere for the heat to go, and the tunnels have already warned up over the decades. (2)
> Conventional air conditioning was initially ruled out on the deep lines because of the lack of space for equipment on trains and the problems of dispersing the waste heat these would generate.
Every solution is a costly compromise. e.g. drilling a new vent to the surface is theoretically possible, but very hard in a densely populated and built-up area.
A lot of comments talk about tunnel size, which is a minor factor: Everything's easier to engineer when you have more space. But fundamentally the issue is where to send the waste heat, not "tunnel too small".