The +3 to attack is important for characters that do more than just damage, sometimes getting that extra 5% to hit with an ability that will cripple a dragon for a turn is much more important than hitting 5% less for on average 4-6 more damage. Also
Mauls/Heavy Flails are dealing 7 on average for every [W] where as Greataxes are dealing 6.5. In the heroic tier, Greataxes getting an extra 6.5 damage on average 1 out of 20 rolls. If you apply that damage over 20 rolls, you only get on average .325 damage. (This increases to .65 at paragon and .975 at epic)
On normal attacks at Epic tier the high crit property catches up, making an average 2[w] attack deal 7+7=14 damage on a maul and 6.5+6.5+.975 = 13.975 damage on a greataxe, almost identical. When an attack with more than 2[W] is used, the average damage shifts even more in favor of the maul.
Other factors include the maul dealing more consistant damage, often between 4 and 10 instead of the axe which is constant from 1 to 12. This means when you do crit, there is less of a chance that you would have rolled maximum damage dice anyways, thus shifting average damage more towards the maul. In addition one of the axe-centric feats, gives the axe user the ability to treate greataxes as if they had high crit, a useless stat. The simelar maul or flail masteries actually add bonuses.
Thus, in a purely statistical perspective, greataxes are actually inferior. Not by much, and in practice there may be instances where an axe is superior. Still, if I was a greatweapon fighter or paladin, I would take either greatsword or maul/flail, not the axe.
