I think the items/levels caps are arbitrary, but the reason that no game lets basic stats get above 255 is that it's an octet of 1's in binary, thus somewhat easier for computers use. the 255 limit is used in internet adressing as well. The addresses use 4 separate numbers (xxx.xxx.xxx.xxx) and no number will ever be above 255. Useless trivia yeah!

255 = 11111111 in binary.