Why is it said tht pregnant woman shouldnt sit directly on the floor on the ground?

Don't know. I am an ob/gyn practicing for 17 years and I have never heard that. I see no reason that there would be any validity to it.
Really? I've never heard that. There is no medical reason, but perhaps because it can be difficult to get up if you are very pregnant and very large. Yoga during pregnancy is very popular and a healthy activity that i'd encourage any pregnant woman to do. Many of the postures take place on the floor.