She wouldn't live up to anyone's expectations.
She can't. Everyone has a different idea about what Tali might look like, it's part of what made her appealling to people in the first place. The not knowing.
Her having a face would just make her just another girl, at least from what I hear. In any case they love her with no face and haven't had a problem so far, see no reason to change it now.
Still it doesn't ultimatley matter to me, Ashley's returning to Mass Effect 3 and she was all the romance my Shepard will ever need*. Tali is fine how she is but if they chose to change her I wouldn't be heartbroken because I have my Ashley.
*and maybe Jack.