Without the atmosphere, the resolvingmpoweer increases in proportion to the size over which light is collected. Its usefulness is limited primarily to bright objects and to red or infrared light, so that the Hubble Space Telescope will continue to be an important facility. The third and unimportant power of a telescope is the magnifying power, the ability to make the object bigger. We can change the magnification by changing the eyepiece, but it will only make the object bigger, not clearer or brighter. Also, light has to pass through a lens, so that the glass has to be very pure and free of bubbles.
Light only reflects from a mirror, so the glass can have internal imperfections.
Mirrors are easier to support since they can be supported across the whole back of the mirror. Lenses can only be supported around the edge. The back of a mirror can have a honey-combed structure that makes it less massive. Less massive, honey-combed mirrors can be brought to ambient temperature more quickly, providing better seeing. Short focal length mirrors are easier to make.