Why do people think Nintendo should ditch the 3DS other than 'its old'?
So they can put all their development resources into the Switch.
The Switch had sold about 10% the amount of units as the 3DS. Why would a developer/publisher prioritise a significantly smaller platform?
Because developers right now aren't producing on the 3DS already. Captain Toad (a port) comes out in two days, Warioware in August (which is also supposedly getting a switch release but I can't confirm that) then nothing announced. That's it. Even the new Yo Kai Watch game is going to be on the Switch.
Except games are still being made and released for the 3DS, because a platform with over 150 million users is going to have a significantly easier time recuperating products costs and turning a profit than a system with 15 million users. You don't need a business degree to know that.
Everybody wants the Switch now though. I feel like people are dropping their 3DS for Switches.