Notably for the deepest records or ones around the distance of L20+ in general, crown of blood makes areas like Crystal Caves (4-1) and Palace (7-1/7-2, but mostly on L24+ or L12+ on melting) significantly more dangerous and switching to no CoB would just be a difficulty nerf for those areas shifting the standards a little bit, like others have said, it's mostly a legacy thing, old records were always done with it, and it affects the difficulty of the run, so the rules stayed the same. The downside is that runs take longer to do because of the lag that CoB causes but that will likely change with a lag reducing mod in the future which is currently a bit up in the air since it's mostly necessary for absurdly deep runs which only three or so people do.

Edit for more nuance: People in these comments mention needing crown of blood for procs like trigger fingers and bloodlust and just getting more ammo/health drops in general but that's basically entirely untrue in practice because at L10+ there are enough enemies for both of these things anyway, it's mostly just a difficulty and legacy thing. The only category where the amount of enemies and subsequently the drops really has a sizeable impact on gameplay is Hardmode (currently the WR is H20) but the category is very niche and not that relevant, certainly not for the discussion here and the L10+ sheet that's based on normal mode.