Even in the mountains in California a bug head net is a staple on my backpacking trips

Is there any chance this is due to a lack of natural predators or something ?

Only on a short term basis. As the snow is melting, this creates a large number of small, shallow ponds where mosquitoes can breed without being eaten by fish. As the year progresses, these ponds dry up, reducing habitat to those permanent lakes which likely have fish present that predate the water part of the mosquito lifecycle. Moreover, there is a lag introduced by the predator-prey dynamics, in particular short lived species like dragonflies and mosquito hawks that predate the flying stage. Thus, it can take a few weeks for these predators to start reducing the number of mosquitoes.

Source: I’m hiking the Pacific Crest Trail and I have been swarmed by mosquitoes on several occasions. Mosquitoes were particularly bad in Yosemite in late June and Washington in early August; mosquitoes are finally letting up now that theses processes are taking place. Let me just say mosquitoes make the most of that interval before predators dominate.