I'll take a shot at this since it seems no one else will.
1. Nothing wrong with the 264 Win, but it has a belt and the bench rest crowd doesn't like belts.
2. 6.5x284 has an excellant reputation for accuracy. (well deserved).
3. Factory rifles in 264 Win usually have twist rates >= 9" so it doesn't stabilize the 140's as well.
4. Not as efficient as 6.5x284, handloads easily equal or exceed factory 264 velocities.
5. Although, properly handloaded to equal pressures the 264 Win is capable of more velocity.
264 just hit a wall when the 7mm Rem was released and couldn't regain any momentum. I can't help but think that the 264 Win when properly loaded in the right (8" twist, 26+ barrel) rifle makes a helluva LR rifle. Just my 2 cents.