And nuns have far less opportunity to get sponsors. Most sponsorship programs are for monks. Also, nuns are called less to give pujas, as people tend to take male monastics more seriously. According to Kim Gutschow, author of "Being a Buddhist Nun", the result of several years of living among nuns in Ladakh, nuns mainly earn money by working people's fields. They're not free to spend much of the day studying and meditating, as they need to earn money for food, or get paid for their work with rice or barley.
Of course, taking money for giving prayers or pujas wasn't allowed by the Buddha. Monastics weren't even supposed to handle money at all. If that's in the Vinaya, it means they're breaking their vows.