In Are Authors Technological Poseurs?, JRuby hacker Charles Nutter suggests that “Good authors do not have time to be good developers.”
There are plenty of counter-examples, but I think the number will end up in the range of several dozen, while the number of great developers in the world is easily hundreds. (Ohloh had over 60,000 F/OSS developers identified the last time I looked, so the top 10% could be 6000 people.)
The nature of the skills are different, though. Writing’s less unambiguous than code, where at least you have a chance of independent and repeatable verification through technical means. It’s not a skill you can practice much on your own into the dark hours, because the purpose of communication is all important (at least in technical writing).
If it takes a year to write a good technical book (and it usually takes at least a year to write a good technical book), how much brilliant code could you produce in that time? Maybe it really is the case that to master a subject, you have to work at it full time — and while you may be good at both, you can only master one. There’s just no time to do both.