The reason they don’t support Monero at the moment is due to a few reasons, one of those being, proton doesn’t want to be a main support for illegal activity which means while they may adopt it eventually, they will be a late adopter. Proton is for privacy, not anonymity.
It doesn’t seem proton is neglecting their core apps, instead it seems they are getting more people, so each service has enough people. https://proton.me/careers