I think it's likely due to inbreeding, and there's multiple breeds, they aren't all the same. The original pet hamsters were Syrians, and yes, ALL the captive-bred
Syrian hamsters came from a single pair of them that were originally-captured from the wild...so it's inevitable for genetic problems to become established.
But I still suspect it's an over-statement to say "they all are predisposed to forming tumors" any more than all other aging mammals are. The only hamsters
that I worked with were Russian dwarfs...I really like them because they're more social (with each other).