To the original question, " Why isn't the 7mm-08 round more popular?" I don't know, but I've read many plausible explanations from marketing to performance versus other calibers.
I can only relate my experience with the cartridge. I bought one of the earliest Rem Models 7's produced and chambered for 7mm-08. It shot terribly out of the box. My gunsmith worked on it and after bedding the barrel, re-machining the bolt jugs to lock up squarely, he gave me a solid 1" gun using the 139 grain factory ammo. [It was interesting to read the post about the poor quality factory ammo a few above this writing; that would have explained a few things, back then.]
After shooting all of my factory stuff up I began to reload for this rifle and was disappointed by the fact that recoil became uncomfortable when I started to approach the upper end of the speed chart for the 139 grain bullet. Again, my gunsmith to the rescue: he tossed me a box of 120 grain Nosler ballistic tip bullets. I've never looked back since that day.
Shooting at near maximum "book" velocity, the recoil is hardly noticeable. In fact, I watch deer tip over through the scope. There is no notable muzzle rise at the shot. I'm using 42 grains of IMR 4064, a CCI benchrest primer, and the 120 grain Nosler Ballistic Tip bullet. I can shoot less than 1/2" groups with this light barreled rifle. My son-in-law has me load it for his Tikka and he shoots cloverleafs. I love the load because I'm a recoil wimp. And I've NEVER [since I began loading these bullets in about 1986 or so] had a deer go more than 15 yards, most just drop at the shot. I guess as a negative, my tracking skills no longer get challenged; but this is a problem I've learned to live with.
I've introduced several young shooters to rifle shooting over the years. One day in particular, I took a young fellow, his dad, and grandpa to the range to help him select the new rifle that his grandad was going to get him for his birthday. He shot my .300 Win Mag, 7mm Rem Mag, .270 Win, .35 Rem, 7mm-08 Rem, and his dad's .30-06. After a discussion about recoil, reloading, bullet selection, performance on game, he selected the 7mm-08. Even his dad agreed that it was a very versatile caliber, especially if loaded in a full size rifle [not my Model 7 'pop gun']. My favorite load shot cloverleafs out of his southpaw Savage; and it earned him numerous accolades from his friends when he took his rifle out with them to shoot coyotes.
I've thinned the vault over the succeeding years but the Model 7, 7mm-08 remains. I'll someday re-barrel it before I eventually pass it on to one of my grandkids. And after years of pondering a change, I'll probably stay with the same light thin tube that makes the gun such a gem to carry in the woods, but maybe add an inch or two in length to burn some extra powder.
Of all of the calibers that I have shot over nearly forty years of hunting and target shooting and all of the rifles that I've owned, this little 7mm-08 Model 7 is my favorite by a long shot [pardon the pun]. So much a favorite that I currently hold an inventory of nearly 2000 of those little 120 grain Nosler BTip bullets that I love to shoot; just to make sure that I'll never run out.