Netflix Originals

this isn't limited to netflix originals, pretty much every tv shows has a yearly wait between seasons
mainly because it takes time and money to produce a new season/show
interviews have shown that itmay even take weeks just to complete filming a single episode

for example, stranger things season 2 released on netflix during october 2017, and now season 3 has just commenced filming, and its most likely due to release in 2019.
they want to maintain quality throughout the production, so they actually dedicate good time into writing, videography and editing.