Let me answer that with another question: why is little Mac doomed to be bottom tier with horribly skewed MUs (with customs allowed)? Or how about why give pikachu a 0-death combo with paralizing jolts, etc? Why let villager plank when the ledge mechanics were designed to stop planking because it was BS in Brawl? Why make characters without jank customs drop in rank?
Overall customs make SSB4 more like Brawl: more campy, gimmicky, polarized MUs, with longer setup time, and less balanced than SSB4 vanilla.
The whole pro-custom argument seems to be "customs aren't OP so people can get used to them (true)" but not taking into account how much jankier they make the game. It was never about "banning customs" it was about "trying customs" because vanilla SSB4 is the original game. Pro customs (not anti-customs) people want to change the rules to make SSB4 less balanced and more janky.