In short, because the wife wants it that way, and usually gets what she wants.
Once more, stereotypes exist for a reason - there is normally some basis of truth to them.
Some mothers are openly critical of their daughter-in-law, not because she is a bad person, but because she wants the best for her son. Most do not do that, but perceived criticism is just as bad.
She has had her entire life to get used to her own mother's ways. You, she still sees as competition for her husband's attention and a threat to her relationship. Maybe he mentions your cooking, or compares her housekeeping to yours. Maybe her Mom had a problem with her Grandma, and the attitude rubbed off. It doesn't take much to put her off them wanting to see you.
It doesn't always happen of course. Many wives and girlfriends are, not exactly fine with seeing his mom, but tolerate it. It sounds like you just got unlucky this time.
Relax about it. Keep in touch with your son and daughter-in-law and just wait. The problem usually sorts itself out in a few years.
When I was married my ex-husband prefered to be with my family. I would offer to go to his during holidays but he just kind of refused to go. He is a db anyway, but I just thought I'd give my opinion. ;)