@geoffc's answer explains why Falcon Heavy over Falcon 9, but the reason for why not any of the other options is likely cost.
It's difficult to say with certainty what the launch costs would be, since costs are negotiated per launch, and are affected by a large number of factors (target orbit, payload mass, fuel costs, ridesharing, etc). Estimates put Falcon 9/Falcon Heavy's main competitors Ariane 5 at around \$178M, Atlas 5 at \$109M-179M, and Proton-M at about \$100M1. Expendable Falcon 9 and Fully Reusable Falcon Heavy both cost an estimated \$90M, and as @geoffc pointed out, FH has greater GTO capacity than F9, so the launch could use some combination of more favorable orbit and extra ridesharing capacity to reduce the effective launch cost.
In the launch intro of the webcast, SpaceX said that Arabsat 6A is going into a GTO with an apogee of 90000km, which is substantially higher than standard GTOs. This confirms using FH for the favorable orbit vs its competitors and F9.
1 this reflects the likely cost as of the time when Arabsat 6A's launch was being negotiated; Proton-M has since been price-cut to be competitive to Falcon-9