A knee-jerk reaction by lawmakers back in the 1950s, that's all. The same reason we can't own fully-automatic firearms without going through an assload of bureaucracy via the ATF. But it seems odd that I can own a semi-automatic version of the most lethal weapon in the world (the AK-47) but I can't carry a small switchblade knife...
It's just as odd that the state of Texas allows me to carry a concealed pistol with 14 rounds of .45ACP in it; but not a switchblade!
Even worse, I live in Texas where--of all places--a Bowie knife is illegal to carry!
It all reminds me of DHS officer I met once, who was authorized to carry a firearm aboard commercial aircraft. He told us one day his bag was searched and TSA found a cigar-cutter in it. He was told he could not take it on the plane. He reminded them that he was armed, and the TSA agent replied that he had authorization to carry the pistol, but not the cigar-cutter!
Cheers! M2
p.s. Actually, switchblades are NOT illegal in every state. See here.