I think because from 1.7 till 2.0 they did a lot of job "hunder the hood" that we are not able to see (for example, volumetric media).
For us, as users, is "simple"
(*) Support of PhoenixFX and add "Inside Volume mode" for CoronaVolumMtl
(*) Add support for VRayMtl and VRayLight
But, behind, there is tons of job.
Also, more features Corona has, more complicated is to add new tools.
I want to explain. If you add a new parameter in Corona, you must to check that this new parameter works with all Corona "universe".
Bigger Corona is (v1.3 was very small, with less maps, less material etc..) more test you have to do.
And yes, it's summer. But I'm sure that they are working like horses even in August ;-)
EDIT: Big things? O god... do I must to be worried Ondra ;-)