The main reason that most professional research telescopes are reflectors is that

Answer;

Large mirrors are easier to build than large lenses.

Explanation; Reflector telescopes have a number of advantages as compared to refracting telescopes and other types of telescopes. Reflector telescopes do not suffer from chromatic aberration because all wavelengths will reflect off the mirror in the same way. The support for the objective mirror is all along the back side so they can be made very large. Additionally, reflector telescopes are cheaper to make than refractors of the same size. Also since in reflector telescopes light is reflecting off the objective, rather than passing through it, only one side of the reflector telescope's objective needs to be perfect.