If Doomsday's earthshaker was parry able, he would only have sweep and guessing as a way of getting past a Wonder Woman, Killer Frost,Joker, Zod, and Batman anticipating a non low attack and getting ready to parry. This would make some MU's like KF and Batman obnoxious for Doomsday. Black Adam and Superman don't have the best lows in the game, but these characters weren't ever dominant due to their "high low mixups". Which is why "he's Black Adam, why would you ever block low?" has been a prevalent statement in the past, but has not seen every character with a parry being in the advantage position in the MU. Therefore I would assume parries aren't shutting down Black Adam. Nor do they Superman. However while Doomsday is similar to them in the fact that he doesn't get by by being a vortex/high low mix up character, his pressure tools(Venom and Long reaching d1 for example) get canoodled by parries. The reason it's amplified in the KF MU is obvious.
If his main pressure tool can be shut down by a parry from mid screen, having earthshaker parry able would limit his versatility. If he dashes in, he'd be left with a grab, an unsafe low, an up venom, or a YOLO supernova in close range IF he feared a parry. If he didn't it's still YOLO anyway.
This was evident when Bane's raging charge was parryable. While it is telegraphed and jump over punishable, especially with air dash characters, it does have uses when on level 3 venom. While there's ways to get around it, and it isn't an end all be all option, it made things very tough for Bane when it was parryable, because at the very least a level 3 raging charge cannot be MBB3'd.
Not to mention it has special properties for being a multi hitting special move that has different high low properties. Flash's lightning kick has multiple hits but doesn't go from high to low. If that's why it's parry able, that makes sense. If it's just a coincidence, it still isn't strange to me to have some moves unparryable. Grundy's trait was a GRAB and was parryable, so there must have been balance considerations during development as to what could and couldn't be parries- even more so when some parry properties like the aforementioned get tweaked to become parry immune.
The fact that all aerial attacks aren't overhead by default also shows that there isn't a one size fits all approach to properties- as ducking Zod balls will get you ridiculed by your peers but ducking nova blasts will get you closer to Billborg.
I never even thought of that being the case.
