Why do all mamas seem to make friends and I can't?

I've never been able to make any friends, despite trying to be more social. I'm usually quite shy and I've tried so hard to get out my shell a little but so far I just haven't been able to click with anyone and usually they stop replying to me or we just don't talk past small talk at playgroups. I'm not sure what it is but it's really getting me down and I feel quite lonely. At the park today there was a group of mums with their babies all chatting about clothes and breastfeeding and other mum stuff and I just felt so sad that I didn't have anyone to talk to about anything like that. I don't have any friends that are mums, and my husband doesn't really understand or care half the time. It also makes me feel guilty that my little one doesn't have any other babies to interact with on a regular basis that could become his friends.