Why can’t we use bitbucket pipelines in bitbucket server?
Thanks for reaching out. Bitbucket Pipelines was designed and built for Bitbucket Cloud (i.e. bitbucket.org). Instead, Bitbucket Server integrates with Bamboo (though it can be used by other CI tools). You can find more information about this topic in this Jira ticket: BSERV-9245.
Hope this answers your question.