I just don't see what is so great about the 25 cal bullets when compared to the heavier 6mm rounds.
You mean 6.5?
Quarterbores have been around in the US a lot longer that 6.5's, so that's part of it.
Remington had introduced the 6.5mm Rem Mag, and Winchester the .264 Win Mag, both of which had lackluster sales and are presently semi-obsolete, despite numerous attempts at revival. They both barked quite a lot, too.
The .260 Rem has faired a little better, but still not a sales leader. If I'm not mistaken, the .25-06 leads it by a rather large margin. This is due in no small part to the fact that the .25-06 simply has more potential due to the larger case.
The 6.5mm Swede has always had some following here, and is a good performer, but has not been chambered by US gunmakers on a large scale.
There are a number of 6.5's in the target world, but these are specialty cartridges, many of them proprietary or totally wildcat.
There are others, like the 6.5 Mauser, 6.5 Carcano, 6.5 Jap, 6.5 Mannlicher, etc., but they have never been picked up by a sporting rifle manufacturer over here.
The appeal of the quarterbores, especially the .25-06, is a flat shooting, fairly hard hitting cartridge that has moderate recoil. The Bob isn't too far behind the '06, the Weatherby just a little ahead. I would wager, though, that the .25-06 accounts for more rifle and ammo sales than all other .25's combined.
I personally love the .25-06, was my first rifle that I bought. I use mine for everything from varmints to elk. It's a great round.