These 'from the ground up' totally all-new-code approaches to DBs are just a scary proposition. Think of the thousands of man-years of effort that went into building MySQL, testing its codebase, and perfecting it's robustness (fail-proof-ness). What does MongoDB bring that couldn't have 'built on top' of MySQL codebase, and used MySQL transational layer as it's underpinnings. Sure, MongoDB gets all its performance ga…
Probably because there has been a lot of development in distributed databases in the last few years. Raft is fairly new, for one. Taking really old code and trying to make it do new things is very very hard. After you've invested all that time and effort making mysql better, it's still a database owned by oracle. Think how Java would have turned out if Sun made it to be cross compiled to C rather than run in a VM.
There are plenty of examples in the Language world also: Go, Rust, etc. All those new languages SHOULD have built on top of Java, for the same reason Mongo should have built on top of MySQL. BTW, Java DID leverage C++. The Java compiler IS written in C. Mongo didn't use any part of MySQL, but Java uses EVERY part of C.