Gmail used to have a single concept, and do very well at that. Someone thought up a vision for how an ideal simplified email client should work, and made that as a web application.

Then the usual cruft built up. People wanted to "add features," not realizing that doing so changes the concept of the whole. The interface became non-consistent.

In general, as companies grow, so does the cruft. Projects are no longer in the hands of a single visionary, but a committee. That committee makes demands of programmers, who graft on the new changes. These accumulate and soon inconsistency is the norm.

At that point, users flee to other projects.