The very simple answer would be: it is easy to attract men to your army if you are winning battles, much harder to attract them if you are losing battles.

Villa had great success in the early years of the Revolution bringing his Army of the North against the forces of Huerta. Huerta was deeply unpopular, and Huerta's men were conscripts. They were mostly placed together in garrisons, so they would not be easily able to desert. Against garrisons with low morale, Villa's mass cavalry charges were effective. The officers of the garrison troops also knew that, if they were captured, Villa would have them shot. It was better to flee.

Villa did innovate somewhat. He would put his troops on trains , and send them where they were needed. Felipe Angeles showed him how artillery could be used. But his opponents changed. He lost the support of the United States, a crucial source of supplies. He blundered in allowing Obregon to organize his army in 1914. Obregon adopted the tactics of the WWI European powers then fighting WWI. Against trenches, barbed wire and machine guns and searchlights, Villa's mass cavalry charges were very costly. He lost some of his best generals. After the battle of Agua Prieta, it was not long before the US Punitive Expedition came into Mexico to hunt for him.

Still, even then Villa was able to rebuild his army somewhat in 1916. But not with volunteers: he was able to do so largely by the same method Huerta had used: forced conscription. When General Pershing arrived in Mexico to hunt for Villa, Villa began to have more and more problems with desertion and bad morale, just as had Huerta. The Army of the North never quite became the huge force it was in 1914. And Villa became, more and more, a leader of a bandit gang.

Friedrich Katz: The Life and Times of Pancho Villa.