Lots of reasons. I get broody...but then I remember:
Pregnancy is knackering for me, and I had hyperemesis in my second pregnancy. Couldnt go through with that again.
Too knackered at the prospect of more sleepless nights and the physical work involved in having babies and toddlers - feeding, lifting, nappy changing, pushing buggies etc.
Practical stuff - at the moment kids have a bedroom each, which would change if another child came along. I also dont want to have to drive a truck around to transport my family everywhere :-)
My career has just got back on track. Another baby would kill it stone dead, and another set of nursery fees when we have JUST got rid of them would be hard.
My body is the best its been in years. At pre-preg weight and have found a sense of style, feel good about myself. Couldnt face getting fat and having to wear leggings for months again.
DH is pushing 50, I'm late 30s. Do we really want to be raising small children into the next decade?
Mainly, if I feel broody, I spend a day with froends with babies/toddlers and it totally cures me. Its hard work. My time is up for all that malarkey.