Simple: those who backed before Epic deal was announced were promised for the game to be delivered on Steam and GOG. Snapshot decided to take the offer to make Phoenix Point an exclusive, which went against what was promised. it upset players who paid for the game believing for it to be delivered on Steam and GOG.
As a compensation for this change, players were offered what I considered a very generous compensation - a year worth of DLCs (which turned to be a full season pass in the end) as well as an additional key for the original platform, once the year of exclusivity will run out, or, if it is not enough: a refund.
I know it sucks for the game to be free of Epic, but be stuck with that platform - I went through that with Hades, which I ended up buying it twice - on Epics and then on Steam’s EA Launch.
Why not be nice and allow players to just buy a game, and play o platform of their choice? Well, it’s against everyone’s best interest. Epic pays a great deal of money to secure exclusivity deals. They do so, to bring people into their platform. Yes, when you buy the game you pay epic the money (though whenever they profit is questionable - from what was revealed Epic usually offers guaranteed sales - so they pay developers for certain amount of sold copies no matter if game sells that much, or not. So to make profit of the game Epic needs to sell more copies then they guaranteed) but more importantly you come to the platform. They want you to use the Epic and they want to keep you there. Personally, I don’t think, it would be fair of Snapshot to take Epic’s money and then migrate all their player base to their competition, even assuming they are not prohibited from doing so. They were offered financial security, in exchange for their player base.
On the other hand, steam or gog have no interest in offering free games to people. You paid Epic, not them. Even if you buy a game on sale, or later the line on platform you favour it is more favourable for them, as they will get money out of it. And unlike Epic, they have a dedicated player base, so they can count on people being tempted to rebuy the game, or wait for steam/gog sale in the future (like I did with Outer Worlds).
There have been instances of Devs offering free steam keys after exclusive period runs out - Klei’s Griftland was one such example, but that was communicated upfront. I have no clue, if Klei’s deal was any different then Snapshots. Either way, in most cases when you buy a game, it’s only for the platform you bought it from: steam, Gog, or epic. People who backed/purchased Phoenix Point after Epic exclusivity was announced aren’t offered a compensation, because they willingly paid for Phoenix Point on Epic store, and they get just that.
Edit: it is a frustrating situation, and one of the reasons Epic is so disliked. Epic offers simply a worse service then either Steam or Gog, and making games exclusive is frustrating - wether you buy early or wait a year. Still, financial security is a tempting offer for developers, on top of a more generous revenue split. After experiencing Phoenix Point release, I am convinced it was the right thing to do - Snapshot needed to work on the game for longer. As to you paying for game - due to how Epics deal works, it is unlikely it benefited Snapshot at all financially - they would have to blow past amount of sales Epic offered to pay for in order to make further profit. More likely, you paid Epic, and reduced their financial loses for buying PP as an exclusive. 
Happy thoughts.