Short answer. Because that's what they had on hand.

Short answer number 2. Because nobody had bothered developing the M1 as a sniper platform until late in the war.

Long answer. Well, nobody was bothering with the M1 until late in the war. Seriously, that's about the gist of it. None of this mess about "Semi autos being inherently inaccurate compared to bolt rifles" (The Russians would like to introduce you to the SVT 40, and some of the lovely and lethal ladies who used it to great effect on the Eastern Front) or pinging clips and such.

When the US entered WWII they didn't even have enough M1 Garands to go around yet, famously the Marines started the war with a large quantity of 03 Springfields on hand, as they had yet to fully transition to the M1. The first priority was getting standard M1's out to all the troops, and ramping up production of them to match the needs of the rapidly growing armed forces.

So, when a sniper rifle was needed, it was simpler to use the existing 03 Springfield platform to use as sniper rifles. They had been built in sniper configurations before, the tech was all worked out, there was a ton of civilian knowledge on how to turn them into scoped rifles. It was easy, simple and straightforward. The M1C and D rifles didn't make an appearance until 1944 and beyond, because of the need to first focus on getting standard rifles to the troops, and the fact that an entire program of turning the M1 into a sniper rifle had to be started up.

The M1 C and D were standard sniper rifles during the Korean War, and served quite well there, which should settle the myth of the weakness of a semi auto platform as a sniper rifle. You can also see the Russians have fielded many successful semi auto sniper rifles, such as the SVT 40, and the famous Dragonuv. The M1 was not a standard sniper platform in WWII, because it had not been developed as such yet, nothing more, nothing less.

Are bolt guns more inherently accurate? On off the shelf rack grade units? Probably. But when you start building match grade semi auto and bolt action rifles, I don't think it really matters anymore. Modern out of the box AR 15's get exceptional accuracy, and it's not uncommon to find or build sub MOA units for competition or varmint hunting. It's not the action of the gun, as much as it is the way it is built.

Here is a good article on the development of the M1 Garand sniper rifle. Note that it wasn't until 1943 that they even thought about developing one as a sniper. The first prototype was rejected because it was an unsatisfactory attempt to use existing off the shelf components that were largely incompatible with how the Garand operated. After the first prototype, the Army developed a series of guidelines specifying how the scope was to be placed, and how the weapon should function in terms of adjusting eye relief of the scope, use of iron sights and ability to easily insert a fresh clip. The ultimate end result were the M1C and D snipers used late in the war and in Korea.

Aside from some of the excellent info already listed here, I've seen no mention of the nature of the Garand receiver's method of loading compared to the Springfield.

The Garand loaded from the top in an 8 round en-bloc clip, which meant that any optic mounted on its receiver had to either sit significantly forward of the action or off to one side. Neither of these is conducive to accuracy, even if an experienced shooter on another platform may be able to adapt to it with practice.

The Garand's direct descendent, the M14, was much better suited to a sniper role (and has filled this role from Vietnam to the present day in US service). It is fed from a 20 round box magazine underneath the receiver rather than on top, so any optic mounted to it can sit in a more conventional location. Two highly accurized versions, the M21 and M25 feature heavily in long range lore, with the name Carlos Hathcock coming up often.

While it's now possible to make a semi-auto rifle as accurate as a bolt gun, there aren't a lot of people who would doubt that a bolt gun can be fitted to a sniper role for a lot less money than a semi-auto. It's worth looking into the M24 program, which didn't come around until the late 1980s, and was based on the Remington 700 bolt action rather than the large reserves of M14 receivers that the DOD already had in inventory.