With how barebones the OS is and how it has 750mb and a CPU core dedicated to it, this isn't even a technical/resource decision. This is someone in Nintendo thinking off-loading all this stuff to a phone is a really good idea and forcing that decision through.
Yeah there's no technical reason the Switch can't support voice chat on the system itself.
It's kinda disappointing that they decided to include the IR Cameras on the Joycon in a weird position that 99% of games will never use instead of putting a Microphone on one of the joycons or the system itself, along with supporting microphones via the headphone jack.
Edit: Of course the whole online system should be on the system itself with the phone app as an optional extra.