Probably for the same reason that all WW2 era NON-US semi-autos are only allowed to compete in the "vintage" sniper event... if you try and compete WITHOUT a scope against the "vintage" rifles it's not allowed as ONLY bolt guns are allowed in "vintage"..but yet the semis are allowed in the "vintage sniper". So the ONLY category you can shoot a "vintage" semi is in the "MODERN" category....which is basically an AR15 match (lite) competition...Which begs the question: why are they not competition correct?
I proposed a rule change to allow vintage semis to shoot in the same category as the vintage bolt guns and any 5.56,7.62x51 and 7.62x39 cal rifle fell into the "modern" category.
I'm one of the oddball types..I want to compete with the weird stuff...it's kinda weird seeing my 60yr old semi listed as "modern" while everyone else on the line has AR "variants" that are scoped and tricked and just meet the "rules".
My petition to the rules committee had no effect..they chose not to change any of those rules this year even though they told me LOTS of people had mentioned similar issues....I'll try again next year.