I didn't really understand the reason until Metroid: Other M. The series started long before regular speech was a possibility in games. By that time, pretty much everyone has their own thoughts on how the character should sound, as well as his/her general personality.
Now there is a new game telling everyone what the "official" character sounds like, and it matches almost nobody's expectations. There is a huge clash with how you have thought, and what is shown in that game. Just like in Metroid: Other M, everyone is upset because the character has, in their minds, been completely changed.