I think in an ideal world, they could have filmed scenes like for season 3 at the tail end of season 2. Those actors plus Brent Spiner were around. But considering they didn't know if they'd pull off the E-D bridge until the last minute, they probably had to keep their plans loose in case circumstances changed. Did they want to lock in how the revival of Data would go that early?
Whereas they were able to get John de Lancie months later again, so that was easy to do. Even then, he went uncredited.