Logistics. How do you build a cloud city on a planet where any trip to the surface is hell. You’d need to transfer straight from orbit to floating, any complementary mission would need to track and align with a moving atmospheric target. And what do you do once you’re floating about in clouds with gaz CO2 your only resource and an opaque atmosphere? Mars is really hard. Venus an order of magnitude harder. Either you’re trolling or this should be in the humour section.