Onward is less popular because the maps and objectives are terrible. They took search and destroy, removed the critical defuse potential, and put no thought into map design. The only reason it was ever playable with /vrg/, aside from Pavlov being even worse at the time, was because most of the players ignored the objective and played it like TDM. The Onward Volk meta is just camp near the objective waiting to shoot anyone who goes near it and hope they don't glitch the code through a wall. Marsoc meta is hoping the enemy team doesn't do that.