Why only people with land/farms could join the army before the Marian reforms? It would make sense to recruit people without land, they would not have as many commitments while on campaign and it increases the drafting pool of the army.
Quite simply, because pre-Marian soldiers were levied men expected to provide their own equipment for the duration of their service. Armor and weapons are expensive, and men without economic support would have an extremely difficult time providing the required equipment. The poor were often levied as skirmishers, but not as the prototypical heavy infantry that formed the backbone of the legion.