That's because of the pants increasing the risk of injury and most BJJ practitioners are NOT vocational/professional/paid to do it, ergo it adds unnecessary risk for very little gain. Besides, ankle locks that target the achilles are more effective in tournament situations.
I agree that elbows being banned would be a good idea for the following reason:
The cuts they cause do not enhance MMA, they just lead to doctor stoppages, scarring, and seeing the carnage during Miller vs Lauzon was a great way to make my gf (and other casual/squeamish viewers) justify opposition to watching UFC.
I think Pride had a better rule set personally, not because I'm some irrational fan boy, but because they seemed to get it when it came to elbows.
I don't have a particular fighter that I am fond of that would benefit from a ban nor do I dislike any fighters that would be harmed by a ban, I am neutral, but I believe elbows are bad for the sport.