In the beginning there were road bikes. Oh sure, there were BMX bikes and English 3-speeds and American knock offs of English 3-speeds but, if you were interested in a serious bike, it was going to be a road bike period.
Then the loonies in San Francisco started trucking old Schwinn newspaper boy bikes to the top of a mountain and riding them down the trails. It was a lot of fun. One trail was even named "Repack" because, after one ride, the heat from the coaster brake would cook the grease out of the rear hub and it would have to be repacked. At some point somebody (Gary Fisher) put a derailleur on one of those frames so that he could ride the bike back up the hill and mountain bikes were born.
So for several years we had 2 styles of bikes with derailleur gears to choose from. Road bikes with 1" wide tires and Mountain bikes with 2" wide tires. (I know the English 3-speeds and their ilk never went away but there were only about 5 or 6 people who cared.)
Then somebody in Wisconsin came up with the idea of tearing up the tracks from an abandoned railroad and replacing it with crushed limestone. Skinny tired road bikes were a little sketchy on the surface but fat tired mountain bikes were overkill. People wanted something "in-between" to ride the rail trails etc. That's when hybrid bikes were born.