Cost, technology advancing at a fast pace, politics, and gov't contracts.
Lever actions worked, but arming every one of the troops was not feasible. Sharps, Spencer, Harrington & Richardson, Winchester, and Remington were in fierce competition. The gov't "let" the makers keep making better rifles, knowing next month there would be something "better"...
Right up until the Krag....The Mausers were a formidable rifle during the Spainish-American War,...as the Krag was a PITA to reload, and prone to FTF when gummed up in the swamps.
Than the Springfield angle, the 03, simply shot better. The ammo was available en mass, and cheaper than the Trapdoor's, Spencer's, Henry's and Sharps'...
The Gov't wanted "standardized" for the grunt troops. But technology was advancing at an alarming rate. There was the Luger, Thompson, Broomhandles, and again, better Mausers, that kept coming...