Who's to say they don't use M-AM reactors in their ships?

We know the B5 station has a fusion reactor, but the station doesn't move under its own power.
It makes sense for the station to use fusion because the fuel is easier to come by and the extra mass of the fuel is not a great problem if you're not pushing it around as you would have to in a ship.

On a spaceship you have to make every effort to reduce mass, for the sake of acceleration. The more mass a ship has the more fuel you need to burn to push it. And you have to carry that fuel with you so you have to add its mass to the ship. Which means you now need to add more fuel to push that fuel, and then ofcourse you have to add even more fuel to push whatever fuel you added. (and so on and so on)

You have the same problem if you insist the EF ships be covered by 10m of armor.

Galen: Hmm? Oh, sorry. I was just trying to work out whether a comment from me right now would be 'most wanted' or 'least needed'.
Expect me when you see me.