Board Thread:Questions and Answers/@comment-30503334-20200617162615/@comment-1327106-20200618033928

I think there are two reasons why:

1) It's specifically the spinal fluid that needs to be ingested, and Eren's spine was still intact inside his body. It was (for the duration he was in the stomach) protected by the meatsack of his body.

2) Eren wasn't dead yet. There's no precedent for Titan powers being split between predecessor and successor, so it would seem the process doesn't complete unless the predecessor actually dies.