The main reason Hamura and his family went to the moon was to watch over the Ten-Tails' outer body. So, how was Madara able to steal the Ten-Tails' outer body so easily? It seems like a plot crack because the Otsutsuki would have been protecting it, right?
It is true that Homura Otsusuki went over to the moon to watch over the Juubi's husk (Ten-Tails' Gedo Body) but then, he most definitely died around the same time that his elder brother did, and that was long before Madara's time.
Apparently, the Otsutsuki Clan that followed Homura to the moon died out and Toneri Otsutsuki became the last surviving member of the Otsutsuki branch family, which explains a lot actually.
By the time Madara attained the Rinnegan and decided to summon the Husk of the Juubi, the majority of the Otsutsuki Clan on the moon, along with Hamura Otsutsuki, had died off. So, it was not being guarded at that time and Madara could easily just summon it.