Why dont MMA fighter`s turn to boxing?

[Leighton]

I've tried both and I found boxing more difficult of the two. In my opinion and experience the sweet science is alot more difficult to master than grappling and take downs.

That probably means you are more naturaly gifted at MMA than you are at boxing. Also the level of opposition you fought in either sport.
Its nice to find someone who has done both sports. How did you find the ground game in MMA. Did it come easy to you or did you find people would be able to manouver there way around to always get the advantage. And when you go back into boxing do you find it hard not to try and throw a knee every time you get a fighter that puts his head down to avoid punches.