The usual software problem
A company builds a product, in this case a very good mail client, and adds a features over the following years.
Then the usual software problem arrives: the product does what it was supposed to do, plus the aforementioned bells and whistles, so where does it go from there? Do you add more features, in which case it stops being a mail client and becomes yet another Swiss Army software product that does lots of things but none of them particularly well or spend the next few iterations doing cosmetic nonsense like Microsoft? When the biggest improvement to the next version is a new interface then you know things are getting a bit desperate.
It's not easy to innovate in email when most of the issues were solved years ago. Mozilla's current product problem is easy to solve as all the core functionality is there and anything else can be done with extensions.