Plus, in the era of DHS, endo steel and XL engines, mechs have other things to use their criticals on. Sure, on a light or medium mech that's got all the weight saving techs and still has some criticals left, you might just toss it on for fun, but such mechs arn't great physical fighters and don't often make heat easily enough to warm the TSM up. On a larger mech, by contrast, it's rare that you've got the space left anyway, or if you do you probaly could have used it better in the first place.
Lastly, as mentioned, most mechs just arn't set up to use it well. There are a handfull of mechs that just have TSM tossed in for fun, true. But most TSM mechs are specally optomized for TSM, with good speed to close with the enemy, a wide range of small guns to help fine tune your heat, and often a mealee weapon (or at least fists). Even if you could slap one on any old mech, what good would it do?