Personally, besides taking away the game time (and fun) of unlocking characters, if the whole roster was available from the get-go, I'd be spoiled for choice.
Not to mention getting a feel for which ones you'll do best with is better if starting out with a few, and unlocking one at a time so as to actually spend a few rounds with a newly unlocked member. If we had to choose from the whole cast at start-up, some characters would be left out for a good while since chances are we'd pick an old favorite or desired newbie and not mess with much else since there is no point in playing with other characters besides getting their trophies.
Of course, plenty of us (myself included) will try out everyone, but I figured I'd just throw that out there.