Both of those products allow for trivial data import/export. What exactly is the problem?

They are closed source. People that are really serious about open source software from top to bottom have issues with dependencies on closed source tooling because it promotes an ethos they don't support.

Ah.. I was hoping there was a substantial practical reason that I had overlooked.

Ah, the classic open source freerider. Only support it when convenient, ignore every other situation.

They are using pull requests on Github, and I don't think that these are trivial to import/export.

This is well-documented[1] in their API. You can list a repo's PRs and the associated metadata/links that make most of the github ui work (code reviews, linked comments, commit statuses, etc).

[1] https://developer.github.com/v3/pulls/

Having a documented API is nice, but I still wouldn't call it "trivial" to migrate out of.

Besides, we don't know if this API is going to stay around forever or whether it will be discontinued. (Unlike, e.g., the git repository itself, which will always be self-contained so easy to migrate away if needed.)