1. NCR already fucked up at the first battle because of >>713928898 the leadership doesn't want an artillery attack because hurr glory
2. NCR only has a few functional artillery pieces, and only a few functional trucks. They are not dispatched to a high risk area like Hoover Dam because the road to it isn't safe.
3. NCR's main fortification in the area, Searchlight, was already destroyed by Legion espionage. Even if the NCR can successfully tow in artillery, it's probable that the Legion will successfully destroy it before the second battle.
4. The NCR's other big fortification, Camp Golf, is defended by incompetent retards that almost always die unless the player completes their quest with enough skill points to get a good ending.
5. The NCR's surviving fortification next to the strip, Camp McCarran, borders the hardcore raider Fiends who are supplied with chems by the NCR's other enemy the Khans. Parking it there would cost a lot of men, and ultimately be totally unavailable in the second battle as the Fiends would attack thereby preventing it from leaving.
6. The Crimson Caravan and adjacent Gun Runners don't want to invest in such heavy equipment, and would sabotage each other anyway (and there is a player quest about this)

And it goes on. The NCR had access to Archimedes but put a retard in charge of it and would rather use it as a power plant against House anyway. It's easier to bring in power armored troopers and have them blow away Legionaires with M60s and flamethrowers. NCR's power in the region was the railroad that the NCRCF Convicts aka Powder Gangers blew up. It's reconstruction is more or less the nation's first job once the Legion is out. Or House's job.