Astronomical telescopes are not just about magnifying, especially when it comes to deep sky observing. A large aperture is important both for collecting as much light as possible and for resolving fine details.
For deep sky observing you want a low f-number for maximum light. But for planetary work you want a high f-number: the focal length is greater so you can magnify more without needing extremely short focal length eyepieces. So different kinds of telescope are suitable in these two cases.