Because Bungie said so.....
.......does not mean it was not used before. In Halo: The Fall of Reach Cortana says that a when the covenant atacked them as they fought at Reach, a covenant plasma torpedo destroyed the Spartan's personal armory. Do you know what this means??? The PoA ( if you dont know what that means you ARE NOT a Halo fan) did have weapons that you see in Halo 2 and 3 but they were melted to oblivion by plasma. That means that Spartan Lasers, BRs, SMGs,ect. were in field use before the other two games so if I see another person say "THER WIL BE n0 BR'S IN HALO REACH BCUZ IT WZ B4 HALO 2 AN D 3" I might cry....... literally. Read the books, learn the canon, and be awsome.....
For all you nitpicky suckers out there, here is a quote
Halo: The Fall of Reach, Pg.337, paragraph 11
"Hull breaches patched- But the majority of decks 11, 12, and 13 are destroyed- that includes the Spartan's weapons locker."
- Cortana
[Edited on 06.19.2009 5:36 PM PDT]