Personally it's a lack of trust.

I've set up databases, I've broken databases, I've restored databases, I've scaled databases, and I've migrated them. Never lost so much as a nibble of data.

My introduction to NoSQL was an article about Mongo losing someone's production data. Not even all of the data - partial data loss, the worst kind of data loss.

I'm just gonna head off that massive headache at day 0 and use an ACID compliant database.

From there it's lack of experience. Same reason I wouldn't decide to write my next thing in a new language unless I was specifically learning that language.