Yeah, I guess it's pretty common, but at the same time 'normal' yuri is still extremely common. On this site the ratio may be skewed though...
Of course there is a lot of reason why this happens, some of which have already been mentioned. Sometimes its for deeply personal reasons, other times it because of simple reasons.
I do enjoy all forms of GB. When its not important at all, when the mc realise they prefer it, or absolutely hate it. Is the imbalance? Yeah in most cases it's the first option, then second, and third is rarely touched (Then there's the 'middle ground' between two and three, where they say they hate but obviously prefer it).
Like I said, there are many reason for GB to be yuri. Sometime its the opinion of the author eg two examples (Straight man now female so they still prefer girls, or its popular, but there are a lot of other different ones). Sometime is just as simple as, female mc is cool but it is easier for me to write a male mind, or this GB story inspired me to write so I'm going to do the same.
I think I've only read a couple GB stories that had the mc like their former gender, (Most of time its just yuri, small bi, or no romance), and even these suffer from the same stuff most GB stories suffer from.