There are plenty of ways to slip Lightning through enemy lines, like Rogue's Passage and Whispersilk Cloak, so you can reliably paint a target on the back of the tables biggest threat
You did choose to run the deck, after all
For example, if you want to make a crazy Timmy or Johnny card, a red rare is very easy
Flash makes it versatile against all sorts of stuff, including notorious card drawing permanents, planeswalkers, cascade, etc
The rules are simpler, the battles are shorter, and it's a lot easier to build an interesting competitive deck due to the game being free-to-play and the availability of booster packs for players who don't spend any money