Re: Like it or not
The Falcon Heavy can get enough mass on a Mars trajectory*, but back when they were planning the Mars 2020 mission the FH hadn't flown. Swapping launch vehicles halfway through the planning phase isn't something NASA do very often.
Also, I suspect they were always going to stick with the same rocket they used for Curiosity. That said, I'd be pretty sure that any Mars missions in planning right now, will be looking at the FH as an option.
* SpaceX claim about 16,800kg for a full expendable launch, and the rover+landing system+cruise stage is less than 4,000kg