One of the reasons is that people can t find a good enough reason to protect them. Well, mosquitoes DO breed there and are also a very important component of the food chain.
Birds find the mosquitoes and their larvae quite tasty.
In turn, other animals find the birds tasty as well. This is a picture of a wetland that became a dumping site for sand, dirt, and concrete blocks. Half of this wetland became buried so that a large structure could be built nearby. But, most of them either flew away or swam to another wetland.