Asura control the Asura gates. That means the Asura can move troops and material around much quicker.
The Asura are also the race with flight. The Charr simply don't have anything to counter the Asura parking a gate above the Black Citadel, out of range of Charr guns, and dropping rocks on it.
The airships seen in the trailer (and helicopter from concept art if they make into the game) look like they're (mostly) charr technology based. And what "race with flight" - the asuran flight is levitation which isn't really mobile, we just have a case of "can be lifted off the ground" and nothing else with them.
Also, in order to use asura gates, they need to have an end-point. Which means they have to get to the end-point manually, then set up a gate, then tune both
gates to the correct frequency. That's quite some time.
Thing is, all six of those opinions will be on how to end the war with the Charr. Maybe one suggestion of negotiating peace, with 5 more positions on how the Asura will fight back.
Thing is, they'll still be in opposition to each other, even if they're for the same thing. If you played Eye of the North, you'll remember when we first meet the asura - they couldn't settle on a decision until the Arcane Council leader decided to veto everyone else's opinion and put his foot down. And back then, they were more united than now because they had the whole case of "we're in a world we don't know, forced out of the world we did know, we can't survive if we bicker" whereas that attitude is now gone and replaced with "we have learned the world and we can conquer it!"
Also, the charr have something the asura do not: experience in war. They've fought for well over a millennium and their society is proof of it. They are the only of the major races to have a nation extending past the in-game map, and for the past 200 years have been fighting a 4-sided war and winning
. Asura got pushed out by the destroyers, their empire collapsed, and charr? They were attacked by the Dragonbrand and Kralkatorrik itself only to shrug it off and keep fighting. Forcefields? Destroy the generators. Golems? Tanks, submarines, airships(?), and superior numbers and tactics. Magitech? Magic and technology - separated but still powerful (hell, even charr use magitech, and you'd know this if you played through the Iron Legion storyline where they have a gun that can more effective destroy ghosts).
And besides that, not all asura would be in the war, but all charr would.