I doubt we'll ever get anything out of tesla other than the "simpler" narrative. There's a ton of possible reasons, including not being able to design a production line for 3 different engines quick enough (RWD, AWD F/R), potential quality control/software development issues, and even a potential 'profitability' issue where they can't make the AWD option immediately profitable at the price they want to charge.
To be 100% fair, the $7500 drops to $3750 for 6 months after Tesla hits 200k. Assuming the worst here, 6-9 months AFTER the start of production, you still likely are going to get the 50% credit. Still a $7250 "cost".