The most probable reason is that below that size of cannon, the weapon loses its effectiveness.
Example: You need x many magnetic rails to make said railgun (which is essentially what this is) move at y km/s.
So if x is < z (z being desired x - magnets), then y is not >= v (v being desired y - velocity).
If y is not at principle velocity, then the bullet loses the high-speed lethality that said massive ship provides.
OR
It could just be to vastly limit the tech so people don't make a hand-railgun and walk around blowing people's arms off and punching a hole through pretty much every armor and material in existence, and also making the use of a saber obsolete, as the gun would blast the blade with such massive kinetic force that it would literally break the wielder's hand(s) (or rip it out of their hand) and take out the saber-wielder.
Two different thoughts, same result.