Obviously Parler didn't consider censorship as part of their threat model, while TPB necessarily had to. Moreover, I'm pretty sure TPB has gone down here and there, probably for weeks at a time. I don't think Parler's problems threat model is that bad; just maybe don't depend on companies that are liable to peer pressure. In the worst case you just get hosting from someone who isn't going to dump you because of social pressure (even going out of country if you must).