I think it's 100% to do with having a family. Like you I have around 3 good friends and 2-3 close cousins. I was a total party animal and extremely sociable until I had DS, now 2, and moved to another country, now I probably go out once a month if that, and mainly just for a meal with DH. Last party I went to was a year ago.
If I think back to those party days though, I think there's a difference between the 20 ish-50 friends, colleagues I used to socialise with and be 'social friends' with, and then the real friends in time of need. For me, it's that middle category of 'friends' you see regularly for different reasons that has dwindled the most, and the real nitty gritty been through it all friends are still there. The middle category is now replaced, if at all, with other mums or other parents, who you don't necessarily have much in common with apart from having kids the same age. That´s what I really miss, the stimulation of regularly seeing interesting people and going to new places with them, just meeting up somewhere on the spur of the moment or after work. Then you get so stilted just being with the family or at work but always rushing back to the family, you don´t have that middle swathe of people giving you new impressions and getting you out there in the world.
I'm clinging on to the hope that as soon as kids get a bit older, you can expand again, but they need so much care and attention that your social life inevitably suffers for a few years. And those party days weren´t all rosy, all that angst over relationships etc, not to mention the hangovers!
In summary, I'm sure it's not because you are not a nice person, it's just that kids need SO much energy and input that there's barely enough personal energy left for one's husband, let alone a wide circle of friends.
So you're not alone and it's not forever (I hope!), this hermit style thing will end!