The other thing to note is that early on the default configuration was optimised to score really well on trivial benchmarks rather than production workloads - including having important safety features turned off.
A crashing MongoDB instance in default configuration was more likely to lose data irrevocably than a MySQL 3.23 MyISAM setup.
(note that while I still don't particularly enjoy using it, post-WiredTiger MongoDB is a different story so take this as a criticism of the people making choices in the early days, not at all of the current state of affairs)