Come on, I get where you're coming from but in this same event we just saw Joe Soto inverted heel hook someone.
https://twitter.com/UFCONFOX/status/795069850955546624
I believe its more that heel-hooks and toe holds have mechanics that seem easy to do and people don't appreciate the intricacy of actually hold your opponents hips in place while you finish the submission. Even top grapplers that understand this, probably would prefer a scramble where they might end up on top than committing full hearted to a leg lock so they kinda let it slip.