This response by u/AisleOfTextusPeach answers your question neatly.

https://www.reddit.com/r/AskHistorians/comments/b9vmvs/in_sapiens_yuval_noah_harrari_claims_that_romans/ek7yzn9/?utm_source=share&utm_medium=ios_app&utm_name=iossmf&context=3

Basically Judaism was recognised as an ancient respectable religion tied to a particular ethnic group with whom agreements had already been made.

Christianity was regarded as a new radical offshoot of Judaism that was not tied to an ethnicity. Therefore the refusal of non Jewish Christians to worship Roman gods was not them invoking a traditional religious obligation, it was a collection of ordinary Roman citizens defying the emperors authority. This was especially important as at the time civic religious rituals were an important part of public life.