Simple, because when you are rebuilding, the last thing you want to do is 1) get your future QB killed while your O-line is learning how to block; and 2) kill your new QB's confidence by getting him sacked, picked and unnessary pressure from losing.
With only a few exceptions, you see far more QB's doing well after sitting a year or two than starting right out of the gates.
Let Chad take the pressure and lumps this season and then our true QB can come in next year (or two) and take over when things have been worked out around him.