"Why is that?"
There's a clear difference between adding new features and fixing bugs. The first is "development" and the second applies to a "stable" release. The stable release wouldn't be very stable if all the development happening on e.g. Ubuntu Touch and Mir landed on users' installed systems.
That's why, once an Ubuntu version is released as "stable", it doesn't get many new features, instead focusing on fixing bugs on the existing packages without necessarily upgrading them to entirely new versions.
All new development happens in the "development" release, which users are of course free to try, with the warning that bleeding-edge software may appear at any time and possibly break expectations on how the system would behave.
Shouldn't development focus on the stable release
Quite the contrary, see explanation above.
or both branch/releases will provide the same amount of fixes?
A stable release will get only fixes, whereas a development release will get both fixes and new versions/features.
See the procedure for accepting a Stable Release Update into the stable release, you will see that a requirement is to have a bug fixed in the development series first, before requesting it be applied to the stable release. Also read the "Why" section for an expanded view of the explanation I gave.