I'd say it comes down to jealousy. However, I told my dad the other day, he said congratulations but has STILL not asked the due date. He's not mentioned anything actually. Quite strange, but Im just going to focus on the people that are being supportive and happy.
Maybe you should just bring up being pregnant to them and just fill them in on what is happening. Then you'll see if they're uncomfortable or rude or whatever. Maybe then you could ask the problem?