Interface is amazingly complicated though, since it has to model all of MtG.
MtGO: old official version, you can buy and sell singles for similar costs to paper
Cockatrice: Old unofficial free version. leaves a lot of rule/effect enforcement up to players manually.
Duels: "new" official version, never implemented complete rulesets, abandoned by devs 2 years ago, leaving collectors who paid good money SOL
Arena: new official version, but with hearthstone model of only RNG card purchases, no singles or resales.
Personally, after putting close to 4 figures into MtGO, then hundreds into duels for nothing (on top of the fortune I've spent on paper), the price fatigue has ruined arena for me, and I don't give them any money.
(to ruin the joke: this is where the famed bitcoin exchange MtGOX came from. It was an exchange for MTG Online that was repurposed into a bitcoin exchange... with that kind of flawless pedigree who could have ever foreseen problems coming down the road? /s)
But for real, the target demographics (nerds who want to speculate and also feel like they're doing something cool) made it the great fit it was.. especially when magic speculators themselves needed a better vehicle for peer-to-peer value exchange.