Ten years of war, a military draft and hundreds of lives = more money. Its been to long and to bloody for that argument to work. The Legion has reached cottonwood cove, destroys Searchlight and Nipton. They don't have them contained. The general morale of the NCR in the area is there losing the war. If you have an airforce and the enemy does not have the means to counter them. You send the air force in, it doesn't matter that its expensive it'll end the war quickly. Hooverdam provides the NCR with electricity and water, its to valuable to hold back Vertibirds if they have any to spare.
It is a classic example of the Leadership not listning to their generals. The people at the front are telling their leaders "look things arn't contained" and their leaders look at the problem and say "well they are still on their side of the River and we control the dam."
NCR can handle the cost of troop deaths and drafting people. A couple hundred even a thousand over ten year, the NCR can deal with that.
Going from a low level war to an all out war is something the leadership does not want to do. They don't even send in the Rangers and Heavy Armoured troops till things finally look like the Legion will finally attack the dam and even then we learn there are far more "chasing after Ghosts in Baja." By then it was to late for NCR to send in Vertibirds and Artillery.
It is like the Vietnam War. America did not want to send in alot of people and money at first. Then it got out of control and in the end they were never able to use full force. When they did it started to work. Nixon bombing the hell out of the north which they did not do for the longest time finally brought the commies to the peace table.
Iraq another great one. America and allies wanted to fight it with as little troops as they could. Things went to hell because they did not have enough people to make it secure, stretched way to thin. Bush finally said "screw it you guys hate me as it is I sending in another 40,000 troops in." People thought it would be a blood bath but in the end it worked!
NCR leadership is not at the point were they feel they need to go all out against the Legion. They already won a major battle and in their mind they have them contained. Going all out would push NCR over the edge money wise. They can budget for drafties and a couple hundred deaths a year but not the cost of all out war against the Legion.
With the Courier's Help the Legion takes advantage of NCR's inability to act to the threat. Caesar strikes before NCR is willing to throw their full weight around. Without the Courier the Legion will wait and delay. They will keep pissing off NCR till finally they may snap and go all out on the Legion. You can only can poke a bear for so long before it takes your head off.