The 1903 became the main match rifle because it was the main service rifle at the time. It was just the tradition. The Krag was used as the match rifle until the M1903 took over about 1909. It was the same story when later the M1 Garand, M14, and M16 all became competition rifles. It was just because they were the current service rifle at the time.
The other fact you have to understand is the M1917 was never meant to be anything but a bandaid. It was never meant to replace the M1903. Everyone knew before, during, and after WWI that the M1903 would always be the main service rifle. The M1917 was just a temporary fix because of the shortage of M1903's duing WWI.
We didn't plan to get involved in WWI, so we were not prepared for it. So when we got sucked in, we had a shortage of M1903 rifles. RIA and SA did not have the capabilities to produce the M1903 in the numbers needed for the expanding Army. Ordnance tried to see if the commercial manufacturers could make the M1903 to supplement the Govt Armories, but the time needed to set up production on producing M1903's would have taken too long. But three US companies were already been set up to produce the 1914 Enfield rifle for our allies overseas. So Ordnance realized really quick it would take too long to make M1903's but these companies were already set up to make the Enfield rifle. All they had to do was convert the Enfield rifle to 30 CAL and they could produce large numbers of rifles really fast. So that is why the M1917 came into existence.
But before the M1917's even started production, it was known by everyone involved the M1917 was only for the duration of the war and then they would be done. So from the beginning everyone knew it would not last.
After WWI, the rifles were mostly put into long term storage for war reserves for a future war. A lot were also sold really cheap on the surplus market too. For instance a M1903 after WWI usually ran $50 to $60. But I've seen M1917's as cheap as $8 or $15. So some M1917's did end up being used in some in matches, but the M1903 was just seen as the cadillac of the day.
The M1917 was always favorably viewed by most who used it. The only negative thing you see on them over and over is the lack of adjustment of windage on the rear sight. You could adjust it with the front sight, but that was frowned upon. Several companies did make after market sights for them that had adjustable windage, and they really improved the shootability of the rifles.
The M1917 was a great rifle, but it was always sort of seen as a red head step child because even though made in America, it was always seen as a British rifle.
So everyone you see back then just called it the "Enfield" rifle. So it was just seen as something foreign.
Maybe a good analogy is how I look at foreign cars. I've been told Hondas are great cars and some models are more American than some of the cars made by Chevy or Ford, but I would never buy one. It's the same thing you see about the M1917 back in the day.
They were just always seen as a foreign rifle and the M1903 was seen as American.
we'll see how this roller coaster plays out.
