Why are Newtonian reflectors more popular than other kinds of telescope designs?

The most important single feature that determines how well your telescope will work is the size of its mirror or lens. Bigger is always better. And because of this, Newtonian reflectors are the cheapest telescopes per inch of aperture than any other kind of telescope. They are very simple to make and require only one main optical surface to 'figure' making them the favorite for amateur astronomers.

The two types you will hear about are the straight-Newtonian and the Dobsonian. Although their optical designs are identical, Dobsonians are made from a lower-quality glass and use a no-frills mounting. For the same mirror diameter, Dobsonians can be as much as half the cost of a standard Newtonian on an equatorial mounting. If you just want to see the sights, you can't beat a Dobsonian, but if you want to do serious photography, you really do need the standard Newtonian design on a high quality mounting.