It took them over 50 years to replace it.
And, it's not
replaced, not yet at least.
Neither the round nor the rifle or carbine using it.
Oh, SIG really wants to see the change, they'd love to sell a couple million units (versus the 100 or so in the current "study"). But, remember the SCAR? It was the new hotness, too. SPIW, fletchettes, binary & trinary proectiles, "telescoped" ammo . . .
All ammo is a compromise, there is no perfect round. Military ammo has even more compromise in it, too.
Eighty-eight (88) years later, people are still second guessing the choice of M2 Ball over .276pedersen (7x51). (It would have been a terrible round for Browning MGs, as used.) Now, we have this new hotness, a 6.8x51, within 0.5% of the 7x51, other than for the chamber pressure. And, no one yet has shown why the 6.8x51 is so much better than the 7.62x51
Compromise is every where. And everyone has both favorites and "anti favorites."