Why do you say that? Ratchet & Clank was easily as tailored to the PS2 as the Jak series and Insomniac and Naughty Dog shared technology back and forth in that era. There's a reason why there were billboards with Ratchet and Clank in Jak 2.
IIRC (and it's very likely I'm not), most of the issue in porting came from J&D constantly streaming with the PS1 co-processor in the PS2, which made it difficult to replicate on other hardware, at least in the case of a port. But I could be mixing all this up. Hopefully they do a good job.