Maybe its because tanks can be a bit 'all or nothing' in SotTR, if your opponents regularly field enough AT you may not get your 'points-worth' on an expensive investment. However, smaller, nippier mg multiple mg armed vehicles can be excellent points-wise as the can throw out a ton of anti-personel fire for few points. On the flipside, if your opponents don't have much AT, a more powerful tank can be effective. Much the same can be said of using mechs, except that you have more flexibility in their loadout given the design rules.
Basically I'd say play using bigger (or more) tanks for the fun of it rather than strictly for their competitative value (now then, where's that E-100 & JS-3?.....).