What's special about it is that you have terabytes of data to keep available, and at scale git does not play well with technologies like NFS or cloud object storage like S3, so each major provider either pays a lot of money to specialized vendors or has homegrown solutions to deal with the problem.

So on top of your usual problems with keeping a cloud service up and running, you also have that git IO problem to contend with, and to rub salt in the wound, that wrinkle also makes it difficult to fully adopt many "standard" cloud architectures or vendors (such as AWS) which work for non-IO-heavy applications: you always have this major part of your infrastructure that has this special requirement holding you back at least partially (and that can hurt your availability for related services which are not even IO-heavy).

(That said, it's hard to guess whether that was the problem, a contributing factor, or unrelated entirely based on the details provided here.)

source: I work at Atlassian (though not on the Bitbucket team) and occasionally chat to current and former Bitbucket devs on this topic.