My only reasoning for this is they may be worried about kids accidentally grabbing switch 1 joy cons and trying to use shove them in with the new switch. ( I know my niece just keeps all her stuff thrown in a box) Also to maybe avoid parents buying the wrong things for their kids.

They likely want to give an easy clear visual distinction between a switch joycon and a switch 2 joycon, until more people move over to the switch 2.