Because they are both getting used to the AL. Time is what's necessary. The Sox are still atop the division at the moment, and with more time in this division, and in the AL, they will adapt to the level of talent that comes with the shift in leagues. Besides, last night we saw what Tavarez can do, as he effectively filled in for Wells for a bit after he was struck in the knee, and did pretty decently.
Seanez is on a short leash, but as of last night he had 9 previous scoreless innings. I wouldn't wave the white flag on them quite yet, because they HAVE shown that they can beneficially contribute to this team's success. It's just been a bumpy road. I was questioning the Seanez move since they signed him, but because I had such low expectations and now that I've seen what he can do in the recent outings, I'm liking it more and more. Just give them time, and if they continue to perform more sub-par performances than wanted, THEN go ahead and crucify them. They've been getting better, so just sit back and wait a little longer.