because using USB mass storage requires unmounting the volume from the system to expose it as a mass storage volume, and is also very much prone to filesystem corruption if something goes wrong during writing to it since it litterally exposes the filesystem. MTP exposes the storage with a much higher level of abstraction (as objects, agnostic of the underlying filesystem type) and in theory is a way more elegant solution. The issue you ask ? Shitty implementations. And also, MTP inherent limitations. Look it up on wikipedia.
I don't understand why phone manufacturers couldn't come up with a more modern and robust alternative... Oh but I do actually: so they could hook you onto a cloud storage subscription instead (thanks apple and google).