Reason 1: It's Easier & Cheaper For Warner Bros To Introduce Characters
Warner Bros and DC Comics are on the right path by introducing Barry Allen on Arrow. If Grant Gustin knocks the role of The Flash out of the park. I don't see no reason why he shouldn't play Flash on the big screen. Having Arrow and The Flash with their own shows gives Warner Bros an opportunity to really established the backstory and characterization. The production cost is cheaper and it gives more time to establish key story arcs. With a two hour movie you're limited on what you can do and what you can establish. However a twenty four episode season can do much more to establish a universe. Arrow so far has introduced characters such as Black Canary, Deadshot, China White, The Huntress, Bronze Tiger, League of Assassins, Deathstroke, Roy Harper and Dr. Ivo just to name a few. If these characters have reoccurring roles or episodes dedicated to a certain story arch it saves Warner Bros time trying to established them on the big screen.
Reason 2: The Fans Want Arrow To Be Connected To The DC Film Universe
Fans of Arrow would love to see Stephen Amell on the big screen. It would be a slap in the face to fans of the show to see two Arrows, one for the big screen and one for the small screen. I think Amell performance as Oliver Queen has been great so far and I have invested in seeing him as the only Arrow. In addition it will be silly at this stage of the game to have a separate film universe and TV universe when Marvel has found a way to connect both worlds. Why have two Arrow's, two Flash's when you can just connect the universes and save a couple of bucks? It would be a smart move to include the characters from the TV rather than introduce a film version and have to go through a whole new backstory and characterization.
Reason 3: Ratings
Imagine the boost in ratings Arrow will get if Ben Affleck, Gal Gabot or Henry Cavill guest star for a couple of episodes. Arrow has already seen the highest ratings of the season thanks to Grant Gustin as Barry Allen. I think with more appearances from some of DC's favorite heroes it will help Arrow even more.
Reason 4: Justice League
Warner Bros and DC's ultimate goal is to launch a Justice League movie franchise that will hopefully have the same success as The Avengers. Arrow is on the right path by introducing The Flash however imagine if they introduce Cyborg, Green Lantern and Aquaman. If they dedicate an episode to each character to introduce them, they can then get a spin off mini series of six episodes to establish the characters. This process I think will speed things up for Warner Bros and once again will save them some money.
Reason 5: The Spirit of Competition
Who doesn't love the spirit of competiton? In my time I have see WWE vs WCW, Sony vs Microsoft vs Nintendo and now Marvel vs DC in live action form. Competition is exciting and it's good business for both parties if it's healthy.