Opportunity cost. You get better "bang for your buck" a lot of other places (e.g. transferable points). Getting to the CBs is nice, but once you hit that "big bang" the incentives are weaker to push further with Delta vs diversifying (especially on the personal card - at least the business card gives you 1.5 RDMs/dollar past $150k), and having hotel points in at least one program is rarely a bad idea.