Well, they had to pay Bruce to do the voice of Alan, which probably cost enough already . . . so if they used Bruce's likeness for Alan, they'd have to pay him additional money for the right to use his likeness.
So, to keep the budget down, I take it that's why Alan doesn't look much like Bruce.
In a way, I'm surprised Mr. Boxleitner participated in the creation of this game, because he lent his talent to the Babylon 5 game . . . and that project turned out to be a no-show. I bet fans weren't the only ones disappointed with Sierra for cancelling the game!