Because Metric Cartridges are cooler. And are avaiable in so many more classic Rifles.
after doing some research when I agreed to buy a x62, I think it is fair to say this is about the most powerful cartridge based on a -06 sized case. Does it make it much better than a 35Whelan? MAYBE a bit... but not much.
near 375 power with about 70% of the powder? About the only one better would be a 375-06AI, or a 375/280... you know, maximize the case.
Oh.. and there is the Kewl factor!!!
I will be hunting with mine this fall.
I look forward to real world experience.