Super heavies are in game as a dead end of tank development that players can explore. I always suspeced that they made into game because most of the players are war machines nuts and ability to play with big, German tank is what makes them more happy then playing with small German tank.
Important thing is, effects of having super heavy tank brigade attached to the normal unit can be somehow simulated in game though the series of stats modifications that in the end, make such unit more or less useless. You don't need special new game mechanics to do it - you just set brigade stats and viola! Big, useless Maus brigade for you.
Siege artillery on the other hand is much more tricky to implement. What exactly would it do, except from exisiting? Would it slow unit attached to? But it was not used directly with some other ground unit, it was usually long range support.
You would basically need completly different mechanics for siege artillery, that simulate fortification/dig-in reduction factor, extreme vulnerability to the enemy attacks, no-retreat mode taken from garrison units (but unlike them it could move normally, instead of using SR only). You would also probably need to change whole fortification bonus system, to actually make it scale in a way that makes super heavy artillery (like railroad pieces or naval ones) much better in fighting heavy fortifications, while normal artillery is still better against more mundane targets (thanks to the better rate of fire and numbers).
All in all, lots of work to set up something that was as dead end of artillery development as Maus was for tanks. No wonder Maus got in, but Dora didn't.
So yes, if you take criteria of "spice unit" as the sole one, you are right. But if you consider other ones, it's obvious why some of the spice got into HoI2 and some did not.
For HoI3, I'm somehow split on that matter. Maus-like units definetly remain "spice" - and game-wise, they can be skipped without much problem. On the other hand, why take away something that is already decently implemented and doesn't cause any harm? Keeping expensive and mostly useless Maus for those RPers makes sense then.