Some of it is genetics. Some breeders have axolotls with those nice, photogenic gills. :happy:
Some of it is environmental. I've read that axolotls kept in lower oxygen water tend to have longer gills, but I don't know if that is proven or just a reasonable hypothesis.
Of course, axolotls kept in a good environment (clean water, safe and comfortable tank, nutritious food) are going to look better than those not so well kept.