Bitbucket's API lacks an equivalent to the GitHub META API. - https://developer.github.com/v3/meta/
Real world example. I have a webhook handler, it works fantastically for GitHub, I use the GitHub meta API to check if the source address of a GitHub webhook is valid. Adding Bitbucket support requires me to hard code whitelist IPs into my code, increasing the maintenance burden imposed by using bitbucket to host my code, and makes it more difficult for me to recommend bitbucket when working with other people.