The more-or-less accepted theory is that, due to the over-reliance on romance and sex, they didn't want to deal with having a dateable short-race character, so Helia got benched in favour of Halsin.
Bears are fine, though, is what one's getting from this.
Regardless, having a, say, Korgan- or Khelgar-like dwarven fighter would make them stick out like a sore thumb with all the oh-so-special companions that is everyone else on the crew. Too "generic", too "dull". Not layable enough or something.
I heard that, but it is a stupid theory quite frankly. For once, my shorties romance just fine and none of them look like children. And halflings, gnomes and dwarves are among the most iconic races in fantasy.
And Helia got benched long before Halsin was a thing as companion. Though his boring character is of course no substitute for a halfling werewolf bard.