Hi there, 

The reason and the answer are not simple, it depends on a lot of factors. The first thing that happens is that new features and choices are taken. Those are coded and tested and take up a fair share of the time in a year. The rest of the time is used for R&D, trainings, meetings, migrations, tests and so on.

Any bug that is reported through odoo.com/help with an enterprise license has to be handled and either needs fixing, improving or is not considered a bug. What remains are community bugs found by the community version users. Those bugs are fixed and checked at a best effort rate. They usually take quite a bit longer. 

It may seem like bugs do not get handled but quite a lot are actually fixed or improved every month. Software keeps changing which means things keep breaking, that is the cost of change. In general things improve year after year though. For the upcoming V12 extra effort and resources where taken to fix more issues than in previous versions. 

I hope this answers your question. 

Regards, 

Yenthe