Because Relapse got a mixed-negative reception from fans and critics. Other than not being as confident in his own opinion and vision at the time... he said R2 was ''more of the same'' - so he probably figured R2 would be even more panned considering it was a more extreme version of similar material.
When I put it like that - it's understandable, considering he'd been out of the spotlight (as a solo artist) for 4 and a half years... to have his comeback album kind of panned, and to then follow that up with a more extreme version of the same thing - maybe he felt it could have killed his career.
Though i don't think it would have. I think he would have gained some vintage era respect for following through on his vision in SPITE of criticism (thee ethos of MMLP) and then Relapse and Relapse 2 would have gained respect, as Relapse is already, and been understood better over time. As many great, polarizing works are.