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.
Because, if I am reading the article correctly, some hydrogen (which has become radiation) will go through you anyway. This kills you. Somehow gathering up all of the radiation and trying to use it to fuel a ramjet is the same thing as trying to make some sort of shield, which the article says would be very difficult.
I think the idea is that it doesn't ever touch your ship--you just funnel it into your reactor directly. (You have to ionize it somehow...) So it's not equivalent to shielding--it's designed to avoid shielding.
> So it's not equivalent to shielding--it's designed to avoid shielding.
Not quite. Electromagnetic shielding is a potentially viable thing, and can be much less massive than material shielding; a Bussard Ramjet doesn't avoid shielding so much as repurposes the electromagnetic shielding you already needed anyway.
Ionizing is not that big of a problem. If you're going fast enough, hydrogen atoms will be ionized by the interaction with your electromagnetic field. If you're not going fast enough, you can shine a maser ahead of you to ionize things in your path (which cuts in to the efficiency of the ramjet).
The article points out some reasons why it might be difficult (not impossible!) to construct the electromagnetic field required for such a ramjet, but yes this point seems to be completely lost on the author. A Bussard ramjet specifically exploits the presence of interstellar hydrogen as a valuable energy resource.
Unfortunately those who have worked out the physics for Bussard ramjets in detail find that they don't really work as an energy source or a propulsion mechanism while cruising. Indeed - more useful as a brake.
