1. It has essentially Earth-normal gravity. Zero-G long-term is a death sentence for humans. The long term effects of Martian gravity are unknown. It seems safe to assume that Venus gravity is fine.
2. It is protected from impact and radiation by an atmosphere in a way that Mars or asteroids never will be.
3. It has an essentially limitless supply of carbon, oxygen, nitrogen, and sulfur available to it.
4. It is reasonably well-positioned for solar power.
5. It is relatively temperate.
6. Low pressure differential between inside a habitat and outside of one means that leaks are less severe and containment breaches are easier to react to.
But there is at least one huge disadvantage:
1. Everything besides carbon, oxygen, nitrogen, and sulfur needs to be imported, either from a fantastically hostile surface, or down through reentry into an atmosphere in a deep gravity well and rendezvousing with an aerostat.
That disadvantage is a pretty goddamn significant one for human habitation.
But it's not a disadvantage for long-term robot probes, and it's... less... of a disadvantage for a minimal-population scientific base.