Green Lantern sells twice as many copies as Flash.
You can credit good writing and blah blah; but all that hinges on Hal having returned. Besides, Wally's spent half of his stories fussing about living up to Barry's legacy, anyway.
Barry's much smarter than Wally.
Do you really need me to prove this one?
Barry stands at the same level as the other Justice Leaguers.
This is not a dig at Wally's abilities as the Flash; he's of the "Titans" generation and is needed there. The Flash is missing its Dynastic Centerpiece and has been for 20 years.
Barry's a unique character.
Honestly, I've been reading Wally as the Flash for 20 years and he still seems generic at best and inconsistent at worst. Barry may be a little old-fashioned but he's a pretty clearly defined character.
Iris is cooler than Linda.
Except for her very earliest appearances, Linda has never been much more than a prop (now she's a local reporter! now she's a famous broadcaster! now she's a med student! now she's the world-expert on superspeed physiology!). While writers have continued to shove their Legendary Love down our throats, we almost never get any sense of why these people even like each other, let alone have A Love That Transcends Time.
Barry Allen's profession.
With a quarter of the shows on TV being forensic police shows, it's time for Barry Allen, police scientist to return.
The Allen Name.
Bart's dead. Yet the Allen name is supposed to be part of the Flash legacy of the future. Without Barry, that ain't happening.
The JLA Needs Him
Wally's saddled with two children with medical issues, and, as pointed out in the most recent JLA, can barely meet his responsibilities to the wider crime-fighting community.
Speaking of Wally's kids, are we going to enter permanent childhood territory with them or are they headed toward Their Tragic End? In entire case, the story is now theirs, not Wally's. It's never good with the Supporting Cast take over a title. Just ask Alan Scott about his dog some time.
The second Barry came back, Central City magically looked liked itself again. Personally, I need that.