Because nobody knows how to make a Bussard Ramjet actually work. From the abstract: "Stopping or diverting this flux, either with material or electromagnetic sh

Because nobody knows how to make a Bussard Ramjet actually work. From the abstract: "Stopping or diverting this flux, either with material or electromagnetic shields, is a daunting problem." And assuming that you can divert the protons, ironically enough, you'd have a serious problem cooling them down sufficiently to achieve fusion (because if you don't, the plasma will not be sufficiently dense to cause a high probability of fusion events, despite the high temperature); and slowing their passage through the ship so they stick around long enough to fuse would create significant drag. And after you have solved the problems of collecting, matching velocity with, and cooling/compressing the plasma, a Bussard Ramjet is only worthwhile if you have an extremely efficient system for capturing all of the waste heat from those processes and injecting it back into the exhaust. Otherwise, at any speed above about .12c, you lose more energy to drag while collecting protons than you gain back from fusing them. Now, there's no particular physical reason why all of those problems couldn't be solved. But if you've already got some way of accelerating to >.5c, and you just need to worry about radiation shielding, building a whole Bussard Ramjet rather than just trying to push protons out of the way with minimal effort or absorb them in some very massive physical shield, is not the way to go.