Because he is under contract through this year. Because while Castellini may have an idea of the man he wants for GM maybe that man isn't available yet. Or as stated above it would be more fruitful to take time to make the right decisions than to just rush to judgement and make the wrong ones now, just to be doing something.
There is really not much that anyone would be able to do to help the Reds this upcoming season, short of the blockbuster (i.e. Dunn) trade. So why shouldn't Castellini save money and just let it ride till after the season, or by waiting till July when the Reds are 15 games out of first and do the big splash then to drum up some interest. I'm not saying that he won't fire DanO right away, but even if he does will that do anything for the 2006 season? I don't think it will.
If MLB would have gotten off of their collective butts in November and approved the sale back then, then yes I think a new GM could have made an impact on this coming season and I would have been one of the blood-thirsty fans calling for DanO's firring before Thanksgiving, but that didn't happen so we have to accept this coming season for what it is and hope for the best.