Okay so basically in the old Insurgency the maps were much smaller and the time you need to get on the objective is fast so if you take some extra time on getting there because you maybe need to wait for smoke or get past a machine gunner you have time to do that and are less likely to miss a spawn wave. Because the maps are small.
In Insurgency Sandstorm the maps are bigger and you actually need to run about 170 m to get to the objective. This takes time, while you spawned the rest of your team is fighting on the point or dying trying to get there, while you are running to the point. But in the time you reach the point the reinforcement wave passed and if you die on the point you missed it.
My suggestion is making the spawn of the attackers a little closer to the objectives but keep the defenders spawn back, since defending should be just as hard as attacking, I know it depends on the team you play with but the defending team tends to win.
I just personally find attacking much harder, and trust me, im experienced.