I think those reasons have been clearly stated.
-There is a lack of agreement on what settings are okay for custom matches (some like swapped weapons, some don't, some want different bleed-out times, some don't). With ranked games, these settings are out of our hands. There is no room to argue.
-Playing against the same people all night is annoying. One of the nicest things about matchmaking that I've warmed up to is both the variety of the opponents, and the variety to the game types and maps. Piss and moan about certain maps or modes, but it shakes things up. Playing the same top-tier maps on the same mode, with the same people using the same tactics gets old fast.
-A problem that grows out of the above is the need to adjust teams every other match as a means of changing things up. The problem is, I like the team I play with in ranked games. I know how they play. They know how I play. During matchmaking, we're able to stay in party chat and just enjoy ourselves as we play (barring the rare exception where someone wants to do some trashtalking). Where the consistency of opponents is annoying given how matches will play out similarly, a consistent team feels good. Likewise, changing up a team to awkwardly sit in silence with three or four people who I have nothing to say to, and know nothing about isn't fun for me. I want to change who I'm going against, not who has got my back.
-Some of you don't have either the Flashback or Combustible maps, cutting the map variety down considerably. Its cool that you're taking a stand against Epic or Microsoft or whoever on DLC principles, but I did buy the maps, and I want to play them. The higher player count of the custom matches often guarantees that at least one person is holding out.
What puzzles me is that there are enough people voicing an interest in custom matches to fill at least one lobby up without the five people not interested in custom matches.