Many even among the elite of grappling don't have a good understanding of leg entanglement positioning, and so they're not very good (relative to guys like the DDS) at recognizing and preventing the entries into those entanglements, or neutralizing the attacker's positioning when the entanglements do occur. When you see guys who know how to stop the entries (Tanquinho vs. Eddie Cummings is a great example of this) or how to neutralize the attacking potential of the positions (Vinny vs. Gordon Ryan is a good example here) they become much less effective.
Like anything else, if you can't defend the position trying to defend the sub is a long shot, and for a long time no one really put a lot of thought into the positioning that set up leg locks. So once guys did, once they developed a deep system for getting to strong leg entanglements where they could maintain control and hunt for various finishes in a systematic way, they started murdering people. It would be like if everyone just let you get to kimura control because they didn't recognize the setups. Well, once you have the arm, *some* finish is probably going to come. You have to defend the entry into the attacking platform. High level guys are really only starting to do that effectively with the leg game in the last few years.