I didn't even know albino penguins were a thing until now so they are probably to rare to be included in the game
White Giraffes are incredibly rare, even in zoos, as are most of the other leucistic and albino critters, so that's not necessarily the rationale. I wonder if they are excluding the albino variants lately because it's not entirely realistic for zoos which emphasize education and conservation to be breeding them en masse like Seigfried and Roy did. A zoo would never use an albino animal in a reintroduction program, certainly.
I think it would be cool to see some of the other genetic variants that don't affect the animals' health or survivability in the wild as much as albinism does (most albinism subtypes have a negative effect on vision, for instance, as well as rendering the animals vulnerable to predation and skin cancer).
They included spotless cheetahs in the game, for instance. I'd like to see the blotched (pseudomelanistic) pattern show up occasionally too. There are other species besides jaguars that have melanistic and pseudo melanistic forms as well. King penguins without the typical white breast have occasionally been seen, as have pseudomelanistic zebras whose stripes are so wide they appear mostly black.
The thing about rare variants in PZ, though, is we speed up time (and the reproduction rate) so much that something that is incredibly rare in real life, even in zoos, quickly becomes common in game when many players select for it.