As far as I understand, until recently Allwinner has been doing kernel development with sole focus to get Android shipped as fast as possible. It is a fact that they did not work to get mainline Linux support for their SoCs.

Developing for mainline might be slower but in the long-run it's better and even cheaper. I think that they are seeing this now.

You can check the slides at https://fosdem.org/2015/schedule/event/allwinner_upstream/ for the current level of support. This is about the community work for mainline support.