Guts doesn't leave many openings, and Cap's own armor is useless against the Dragonslayer. The crossbow could still penetrate Cap's armor, but would mostly just be flesh wounds and nothing fatal unless Guts could get him in a vital point or the head. Neither of which is too likely due to Cap's enhanced reaction times and his shield.
The main thing is, Cap's greatest defense is his shield, and it's also his primary offensive weapon. For Guts, his greatest weapon is the Dragonslayer, a massive sword. It's also his most powerful defense against attacks due its great size and thickness. The primary reason for this match is because it kinda fits the credo of the Strongest Sword vs the Ultimate Shield, and in this case, both are used for both Offense and Defense.