Short answer?
For an equivalent fuel expenditure:
Lighter TDs win against medium armour and lose against heavy armour.
Heavier TDs win against medium armour more slowly and win against heavy armour.
Why would you risk going for a medium TD build when the opponent can go for a heavy tank and render them ineffective?
An example of a light TD done well is the Puma. It's indisputably a light TD: it specialises against vehicles worth 100 FU and less, and you'd never think of the Puma when deciding what to build against an IS-2.
But an IS-2's worth of Pumas does beat an IS-2.
How?
- It's fast enough to close in on the IS-2.
- Its smoke canisters allow it to survive fairly well against an IS-2.
- It has a powerful veterancy ability it can use to stun single targets.
- It has a close-range penetration spike that allows a group to out-DPS an IS-2 at point blank range.
If light TDs are to be truly viable in the era of heavy tanks, they all need what the Puma's got: utility abilities that are good at controlling a single target, and more importantly close range penetration spikes.
They shouldn't be better than their heavier counterparts, but they should win in numbers.
Otherwise why risk building a tank destroyer that loses to a generalist?
But a unit as fragile as a su76 needing to close in to heavy tanks is just trowing it away. Its got no turret low health. The stug wont fare better. Let alone them flanking without a turret is pretty hard to pull of.
The twp can control a heavy somewhat. Extra sight with tracking can help secure a kill even on a heavy. Imo they already have something that can be off effect vs heavies.