The Krag was brought into service long after the Trapdoor 45-70 rifle had become obsolete, and it was something of a stretch for the Springfield Armory to produce: they'd never had to make guns out of hardened steel before, and had to hire some metallurgists to show them how to heat treat carbon steel, as well as build a couple of new shop buildings. Even though it was a huge improvement on the black-powder single-shot Trapdoor rifle, however, the Krag produced in 1892-98 was still behind the European arms design. When American troops came up against Spanish troops in the Spanish American War, they discovered that the Mauser-equipped Spanish could fire faster, reload faster. The captured Mausers showed clear superiority: they could be loaded quickly from a stripper clip: the Krag had a somewhat awkward side-loading door that accepted loose cartridges. The Mauser was strong and rugged, could use a high-pressure cartridge: the Krag's carbon steel could handle the pressure of the 30-40 Krag cartridge but the single locking lug on the bolt and rather brittle receiver couldn't be relied upon to do more.

The Ordnance Dept quickly set about trying to emulate the Model 98 Mauser without having to pay Paul Mauser ( he did hold some US patents). The result was the Model 1901, which was modified after a review and became the 1903. It was still, basically, a Mauser: front locking lugs on the bolt, safety locking lug on the side, big receiver ring, bolt cocks on open. It also preserved some Krag features- for example, a cone was cut in the breech of the barrel to make loading and firing single-shot possible. (This was something that the Germans had simply dropped, assuming that once a soldier was able to load from clips, he didn't need to handle single rounds.) The design was really not much of an improvement on the Mauser. But the Springfield armory had at this point a mania for precision and the produced rifle was in some ways better than the usual 98 Mauser- certainly the armory never made a rifle with better fit and finish, and the cartridge, the 30-06, became one of the longest-lasting cartridges.

Why didn't they choose the Enfield? They had a chance: Canadian-born James Paris Lee , who designed much of the first Enfield, beforehand designed one of the breechloaders the War Dept. used in the US Civil War , and if the War Dept had decided to fully fund his latest design in 1872, instead of that of the Springfield Armory's own Erskine Allin, it's possible Allin's Trapdoor rifle would have been instead Lee's design , and if the Armory had been willing to make such a big change, something with a magazine could have resulted. The Short Model Lee Enfield would probably have worked as well as the M1903 for WWI: it was for a lower-pressure cartridge and less accurate but accurate fire at 1,000 meters was not that important... it also could have a higher rate of fire than a "cock-on-open" bolt action. But the Ordnance Dept in 1900 clearly saw their task as duplicating the best features of the best rifle, and doing it even better. Maybe because Teddy Roosevelt was President, there was a lot of hubris available.

Of course, the US would adopt something called an Enfield in 1917. Remington had contracted with the British to make for them a Mauser-influenced variant on their Enfield in 1914, and when the US also found itself at war it also found that the lovely and intricate M1903 Springfield was not easy to mass produce in millions. Remington , Winchester and the Baldwin Locomotive Works were contracted to build more of the simpler Enfield variant, sometimes called the P-17 Enfield. It was found to be perfectly serviceable- Sgt Alvin York had one. And eventually it was found that reliability and rate of fire were more important for a military rifle than almost anything else. Although the M1903 was a superbly made rifle and very accurate at long range, what really mattered was how fast a gun could shoot at closer ranges and how much ammunition could be supplied for it. The 1903 became the simpler, rougher, more rugged ,cheaper-but-still-effective 1903A3...and then there was the Garand...and then the very-cheap-and-very-effective Kalashnikov.

Raber et.al.: Forge of Innovation: An Industrial History of the Springfield Armory