The game may not be, but the vetern players are...
Basically, the double handful of players that are advanced enough (and still playing) to make use of larger bots, isn't large enough to warrent the introduction of those bots?
Add the larger bots to the game, to provide a new goal for corps. Make the extension requirements high, the kernels rare drops out of beacons, and production require the new rare mineral off gamma; all those things will ensure the bots do not go mainstream quickly.
Another thought, is MK III bots, that are like MK II, but have 5% more reactor, cpu, and acc; and a little heavier with more cargo. MK III bots would not be factory buildable, only prototypable (making high material dependant and very limited production) and they require a MK II bot as a component. The 5% is just enough to make some really tricky fits, but still only if the player has some high extensions. And the increased stats to value ratio is so small, that only a very few players would even see any benefit from them. The point of the MK III would give vetern players something to shoot for, that wouldn't over balance the game. Recall when MK II's first came out, no one used them because they benefit to value ratio was too small. Now we are seeing them much more often, as players have the NIC and skill to use them more effectively, the same would happen with the MK III.