I remember the dev answered this during Q&A.

They said what if 4 players queue at the same time with all role option selected, the algorithm wouldn't know which role to pick for these players in order to pair them in a group together.

A very silly answer in my opinion. They can always prioritize DPS > Healer > Tank (For example).

Another thing that came up was item level. If you were to select all 3 roles, the item level check wouldn't work properly as it will only check the item level for the current job you're on. They'll need to update how duty item level check works.

I would like them to implement this at some point.