Their private lives ended very publicly.
And not just because he was a famous actor.
Because they also seemed to have no regular family or friends that gave a damn in return..
I have no inside knowledge, but there have been several celebrity deaths where the celebrity married someone much younger, and that person isolated them from all their relatives and friends. One of the more infamous ones was Casey Kasem.
Whether done to make sure the will left them all the money, or other reasons, I have no idea. The fact he had mental deterioration and nobody visited would indicate she was the sole caregiver.