The reason is the quality of the lenses. Better optics, generally makes for better viewing, though there are many different types of binoculars and telescopes. A point in understanding what makes a good pair of binoculars for astronomy, is to consider how they work.

The lens has light going through it. The bigger the lens, the more light, and thus more detail it allows. As the objects in space are so far away, they need good optics and a big lens to be able to show us in detail what we want to see.

At this point, you may want to consider just how big a pair of binoculars you want to buy. Binoculars come in various sizes. Some are small and portable, while others can be as big as a telescope! So, consider just how portable you want your binoculars to be.

If you are only taking the pair of binoculars to your backyard, then choosing a larger pair of astronomy binoculars may be the best way to go. Another point is the weight. Remember, you will need to hold onto them, unless you get a tripod. In such cases, it becomes a fine line, whether to buy binoculars or buy a telescope instead.

Binoculars range in price, and you can find binoculars for under $100 to several thousand dollars. A good idea is to try and find a pair of binoculars with lenses above 60mm. This will generally have enough space for enough light to travel through, and give you good results.