Because one, Captain America's durability is, surprisingly, worse than Soldier 76.
Two, he has dodge point blank gunfire, throw his shield to save someone from bullets, even blitz other gun men before. The only possible reason why he has to take cover or block is that the gun men are highly superior than even the Hydra, they use lasers that move so much faster or it's a whole army of armed men that he had to deal with alone instead of a group.
Soldier 76? There's another possibility that he can quickly svlave her by, in a split second, going over that 2 metres before the grenade detonates and he got caught by the explosion. But, why I'm against completely HHS+ is because Captain America, himself, dealt with gun men by himself by blitzing them, he caught a missile, stop point blank bullets, blah blah blah, seen legit in comic pictures.
Three, Captain America showed many speed feats that has continuously been jabbed into the minds of readers and fans while only one feat for Soldier 76's speed is in his own Hero Cinematic...which is the grenade. (Don't bring up the gunfire, the minigun was at far range and it's best for damage and rapide fire, instead of accuracy. Same applied for the guns on the roof. That's why he aim dodges.)
Four, The cinematic skipped to the time where Soldier 76 looked back to the girl before the next frame was explosion. Yes, the timer was going off but, do you know what time was that for an absurd type of grenade that beeps?