Why is this? I can tell you it isn't because they are too slow, it is because it doesn't do !@#$ once a turret comes up or it has to engage an army.
The Oracle is better at engaging a Turret or army than an equal cost/supply of Mutalisks, provided they have energy.
Buffing Oracle speed is akin to giving Mutas increased regen. It makes the units stronger at the only role they're good at.
I proposed reducing the stats of the Oracle by a third.
It would have made the Protoss spend a bit more time building the extra Oracles (weakening proxy Stargate), but given them an increased energy pool and health per cost, as well as making each Oracle loss less of a big deal.
In the beta I used them like a flying Hellion, they are great for killing Zerglings trying to snipe an expansion, or cleaning up a Marine drop, probably better than any other unit for these roles.
But they obviously have their weaknesses.
The Mutalisks sucks the way it is right now.