Perhaps campaigns feel that right now is about the time that donors (and campaign committees) are beginning to make the triage choices, deciding who needs their money, who can probably do without it, and who is such a long-shot that their donations would be a waste?
The DCCC just recently released the seats it would be buying add-time in, for instance (though I assume the DCCC probably has its own internal pollsters).
This is not that far from the truth, really.
The DCCC seat-map purchase is more bullish than what I'm seeing right now, but that's to be expected at this point, since ad buys can be changed as conditions do, and right now you want portray a message of confidence in your planning.