Why are there no artificial wombs yet?
It is impossible at present to reproduce all the conditions of a womb.

A womb is more than a warm fluid-filled home for nine months. A few days after fertilization, the trophoblast portion of the conceptus attaches itself to the uterine wall where it develops into a placenta and umbilical cord while the rest (the epiblast) becomes the baby.

Throughout the pregnancy, the mother delivers oxygen, nutrients and hormones through the her blood flow to the placenta, while carrying away waste. The womb also prevents infection of the placenta and amniotic fluid. 

In addition, the placenta contributes to amniotic fluid production. Production and reabsorption of amniotic fluid is a very dynamic process; it isn’t just there. It is estimated that more than 95% of amniotic fluid turns over on daily basis, and the fluid composition changes throughout pregnancy.  

While cell cultures of the womb's endometrium have been grown, there is nothing artificial which is close to this kind of environment yet, nor is there likely to be in the near future.