Because Tony Allen earned Daniels' minutes. TA comes into games and simply wreaks havoc, which is something that Marquis hasn't been able to do. Allen is a better defender (he did a great job on LeBron), and on offense he can provide a spark if the team needs one by getting out on the break with Rondo and finishing.
The main reason why we brought Daniels in was to be a better, more under-control version of Allen. Daniels also has better handles so management thought he could play the backup pg better than TA. As it turns out, we don't really need a backup pg right now because Rondo plays the whole game. Also, Daniels' defense has been unimpressive for the most part, and he failed to make an impact on most games he entered during the regular season. TA is showing game in and game out that he is an IMPACT player. Lastly, TA's biggest flaw is that he's prone to making stupid mistakes and turning the ball over, as opposed to 'Quis who plays more conservatively. By and large this year, Allen has stopped making those mistakes and turnovers, which combined with everything else makes it a no-brainer to give Tony the nod instead of Marquis.