Probably because the effectiveness of BJJ in the UFC is wearing off. MMA guys are getting good. Many of them can effectively stop most takedowns, and their boxing skills are better as well.
I would say it is balancing out. The bjj guys are learning that they need to actually learn takedowns. (In fact my instructor has been learning judo and forcing us to work takedowns every class now). The strikers have learned they need to learn how to deal with takedowns and how to stand back up when they do get taken down.
It comes down to this. A striker can not ignore the ground. A grappler can not ignore a good striker who did not ignore the ground.