Two accelerated proton beams is much harder to perform experiment, than having stationary protons in chamber, and single beam of protons.   We need to see, detect, "catch" these newly made particles, and keep them in magnetic traps (so they won't annihilate) for further experiments..   It's even harder to describe annihilation process of proton-antiproton. Their acceleration also matters. Accelerated antiprotons colliding with stationary protons will produce something else than stationary