[ch_123]

Any one of: the risk of a migration going wrong is too high, application or infrastructure compatibility issues, or the cost of retraining staff who work with the database to work with something else.

It is possible that there are simple solutions to these problems, but the perception that they are serious will turn companies away from a migration.

[bux93]

I think everybody has a horror story about code depending on the empty string and NULL being the same, silly stuff like that can trip up migrations.

The main reason I'd say is that there's no functional benefit to ripping out a database and replacing it, so there's always something more important to do that actually drives revenue.

I'd argue that postgres brings with it substantially lower risks in terms of license compliance/audits/price hikes. Not sure if that can drive a migration, but it should be reason enough to select open source for new projects.