Is it just me or does a falcon heavy seem like a bit of an overkill to launch Arabsat 6A?
Falcon Heavy - GEO payload: 26,700 kilograms
Arabsat 6A - weight: ~6000 kilograms
Wouldn't some other launch options be more appropriate or is there something about the launch of Arabsat 6A that needs the additional power?
@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.
[Edit]
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
6000 Kg is actually pretty big. I am sure SpaceX offered some discounts to attract a customer for Falcon Heavy.
So I will ignore the other options (Atlas 5 with side boosters, Ariane 5, or Proton) and focus on why not a Falcon 9.
Falcon 9's numbers are lower to GTO, Wikipedia has it around 5500kg reusable. 8300kg expendable, and SpaceX's point is, if you are bigger than a Falcon 9 can handle and still land, better off moving to a Falcon Heavy.
Also there are different GTO orbits, the higher the 'energy' the less work (aka burned up fuel) the satellite needs to do to get to a circular GEO orbit. SpaceX does aim for a lower of the set, usually to allow recovery.
I know the SpaceX website does list the Falcon 9 as having a 8,300 kg payload to GTO Falcon 9 GTO but the wiki for Falcon 9 launches also shows that the Telstar 18V / Apstar-5C launch on Sept. 10, 2018 was 7,060 kg to GTO and a successful drone ship landing was achieved with the Falcon 9 Wiki-Launch 61. This would be a higher weight than the Arabsat 6 and also a GTO launch that did a successful drone ship landing so it was a reusable launch.
The Arabsat 6 also uses powerful hypergolic main engine to boost to GEO from GTO according to Nasaspaceflight. There is no mention on the peak it will be sent in GTO but the satellite using those engines instead of electric seems to point to it needing a lot of thrust to circularize it's orbit.
Unless there are some other specs that show this is being put in a different GTO orbit or higher it seems like the Falcon 9 reusable could launch this satellite.
There must be another explanation such as SpaceX discount to use this as a Falcon Heavy qualification flight for the USAF or something else.
Looks like they just need to continue Falcon Heavy testing, and since the first test went almost fine, they decided to take some payload now, just to pay back the costs. However, risks are still too high (up to 10%, on Elon Musk's estimation) to wholly pack it with expensive electronics, so, a single autonomous Arabsat, to move itself to the right orbit if something goes wrong again, plus some ballast mass is quite a cautious and balanced decision.
While they 'could' have moved this payload to a F9, I think there was a secondary reason not to. SpaceX want's to use bid the FH on military contracts, and have even signed up some... but there is a big caveat on those contracts. They have to have three successful flights before they can launch the more expensive military ones.
So Arabsat is 'demo 2' and the STP-2 flight next June is not a large high value military flight (though it is being flown under Air Force contract) but a many smaller 'throw away' bunch of ride share sats. (Not saying Air Force would be happy if it fails, other than some political monsters, most involved are rooting for success) After that one will be the first 'real' military contract sometime next year.
