If you have a medical condition like high blood pressure and diabetes,you will require constant monitoring of your vital signs like blood pressure,which may not be feasible under water.
If you cannot get in and around the pool on your own,are not comfortable with water,then you will be excluded.
A mother with a big baby may be allowed to labour in water but asked to leave the pool to deliver on land,due to the risk of shoulder dystocia.
If during labour,liquor is meconium is stained.
Once laboring women are given this option,it will become popular.