Why wasn't Captain America "enough" for Colonel Chester Philips?

Phillips didn’t want to send Steve off on the tour to get people to sign up, his plan was for Cap to go to Alamogordo to try and reproduce the serum so he could create the army he wanted. It is Brandt that decides to send him off on the tour to get more people to sign up/war bonds, Brandt’s comments immediately after the part you’ve quoted makes both his and Phillips motivations clear.

Senator Brandt: With all due respect to the Colonel, I think we may be missing the point. I’ve seen you in action, Steve. More importantly, the country’s seen it. Paper. The enlistment lines have been around the block since your picture hit the newsstands. You don’t take a soldier, a symbol like that, and hide him in a lab. Son, do you want to serve your country on the most important battlefield of the war?

_{Captain America: The First Avenger}

He’s not enough because one man is never usually going to be enough and they’re still not sure how good the formula is. But even if the serum was excellent Steve can’t be everywhere in the war at once. Phillips’ plan was to recreate the serum using Steve’s blood and create an army that would all but guarantee the victory.

Brandt came along and suggested instead they use Steve to get people to sign up as more men would help create the victory. He then later used Steve to get people to buy war bonds to help fund the war.