Mystery. Or, to put it another way, which is more interesting: Samus before Other M, or Samus IN Other M? Sure, Tali wasn't the most fleshed out character in the first game, but you had little hints. And you wondered what she looked like, what the deal with that mask is, what's the story behind her people, and the mystery was that much more interesting thanks to Quarians being such a minority. Her character is probably liked because where other characters gave you answers, she begged more questions.
Alternative 1, shes a bit of a woobie
Alternative 2, as it I heard it so succinctly put once, "Dat ass"