Same as the reason all other once-traditional communities rallied to anti-Christ Rome:
Legalism.
When the importance of legality overtakes the importance of doctrine, the result is scruples.
Because of too frequent contacts with Rome, combined with having given deference to Lefebvre while he was alive (but never really having been comfortable or confident in his hard line statements), the scruples becomes a mania, the resolution of which only an accord (written or de facto) can heal, and the achievement of which determines damnation or salvation.
This is how the SSPX got to where it stands today.
Makes sense. I think it was Bp. Fellay who said that they run the risk of schism if they stay too long on their own (or words to that effect).