Why was it so hard to take a picture of a black hole?

The picture that you’ve already seen is of M87*, the supermassive black hole (SMB) in the middle of the galaxy M87. In balance, it was easier to image M87*, a black hole a thousand times bigger and two thousand times farther away, than Sagittarius A*. We were almost guaranteed to see that hot cloud of gas all the way around M87* because, in addition to all the other craziness going on, black holes bend light around them.

Source: www.askamathematician.com