I think the common player primarly has the the California Sea Lion in his mind when thinking about a sea lion species.
It's not so much that. The main point that dominates the majority of wishlists is whether the animal is widespread enough to be versatile. Certain niche species do have their place, like the proboscis monkey, but in general the most requested species are those that appear in zoos all over the world. Good examples are the capybara, Asian small-clawed otter, and the California sea lion. All three species are common in American collections, European collections, Asian collections, and Australasian collections (at least in the past - Australia and New Zealand both tend to focus on their native pinnipeds more nowadays), which is where the majority of the community comes from (I'm only excluding South America and Africa because a) I don't know as much about their zoos, and b) I don't know if we have many members from either continent).
Steller's sea lion, if I have my information right, occurs in some North American collections, but you'd rarely see them anywhere else. California sea lions are simply more accessible for zoos, and one of the primary drivers for players is being able to create zoos that might exist in real life (and for that, we need species we are likely to see).
