I don't have any test results. Not something I've ever looked at.
If I was going to guess I can think of two possible reasons - but without any actual research I could well be wrong:
1) The visual weight of the letters
The visual weight of the upper case letters are usually roughly in the centre of the letter. The visual weight of lower case letters are usually in the lower half of the letter.
This may cause folk using the keyboard to automatically tap slightly lower when you switch to lower case - causing more mistyping.
Admittedly since it's a virtual keyboard you could adjust for this in software if everybody does it. However I imagine that there would be enough different usage patterns that this may not be simple.
The upper case letters are bigger and easier to see and read.
3) The flicker distracts
Maybe the state change causes people to pause to grok the change in appearance.
I could probably make up more possible reasons - but without research I'm basically pulling ideas out of my arse.
I wouldn't at all be surprised to learn that there are a bunch of solid pros and cons to both approaches and Apple & Google have picked a different balance between them.