Because there's no such thing as too many aquatic races.
3 outsider HD, the water subtype, a decent amount of natural armor, an okay swim speed, and a horrible land speed. +2 to all stats other than dexterity and charisma make for a balanced albeit unimpressive stat array.
Apart from this, the only notable feature is a triton's ability to cast Summon Nature's Ally IV once per day, which I admit opens up some pretty good options. Brown bears, tigers, and deinonychuses are all very powerful beatsticks, medium-sized elementals come with some pretty interesting traits, and unicorns are basically walking SLA-batteries (which is rare for SNA summons). And that's before going into earlier lists: at low levels 1d4+1 crocodiles are going to ruin any melee character's day. The CL is an impressive 7, meaning anything you summon will stick around for well over half a minute.
If it weren't for the summoning ability, these guys would be an easy -0. As it is right now, they have the ability to absolutely dominate one battle a day, while being near-useless in other encounters. Keeping in mind that advancing tritons anywhere is going to be very hard, I'll put down a +0 here: do discuss.