Up until 10.1, MariaDB used Percona XtraDB as the default 'InnoDB' storage engine. From 10.2, InnoDB is the default. Why was this change made?
Answer Answered by Ian Gilfillan in this comment.
Keeping InnoDB (or XtraDB) up to date with MySQL (Percona) is a complex task. It took us more than half a year to migrate from InnoDB-5.6 to InnoDB-5.7 in 10.2. Doing it again for XtraDB would probably have required only slightly less than this. For us to embark on such project, it must bring significant benefits to our users.
XtraDB had many great improvements over InnoDB in 5.1 and 5.5. But over time, MySQL has implemented almost all of them. InnoDB has caught up and XtraDB is only marginally better. Not enough to justify a multi-month merge that would delay 10.2-GA for everyone.
In particular, the only real improvement that XtraDB 5.7 seems to have is for a write-intensive I/O-bound workload, where innodb_thread_concurrency control is disabled.
With a proper innodb_thread_concurrency, XtraDB is only marginally better. We didn't want to delay 10.2-GA by up to half a year for the sake of those few users who have write-intensive I/O-bound InnoDB workload and don't know how to configure innodb_thread_concurrency.
Note, we still consider incorporating XtraDB optimizations, but as patches, rather than XtraDB as a whole, which no longer has numerous all-over-the-code improvements.