Their website talks about some of the challenges of getting a picture of a black hole: https://eventhorizontelescope.org/home

All of the data used to generate the image was gathered by around a dozen different telescopes around the world. The black hole itself also needs to be in a specific configuration in order for us to be able to see it. It needs to have an accretion disk that's generating light. It needs to be sufficiently large or close. And it can't be obfuscated by other astronomical objects like stars or nebulae. This black hole itself is hugeeeeee and far. It's about the size of our solar system, but it's ~52 million light years away.

Well, we have in the same sense that you take a picture of bacteria every time you take a selfie: there certainly are lots of pictures which contain black holes that are too small to be seen in them.

In line with that analogy, the basic difficulty is simply that they are tremendously small compared to the sizes of galaxies.

Here's an NY Times article about the project:

https://www.nytimes.com/2018/10/04/magazine/how-do-you-take-...

The author, Seth Fletcher, also wrote a book about it, "Einstein's Shadow: A Black Hole, a Band of Astronomers, and the Quest to See the Unseeable", if you want more details.

https://www.goodreads.com/book/show/36300597-einstein-s-shad...

There have been plenty of indirect observations such as their jets, stars in very tight orbits around Sagittarius A*, xray binaries etc.

This is a direct observation of the immediate environment of a black hole, i.e. its accretion disk and other light bent around it.

This is as far as I understand the first picture of a black hole shadow, that is the dark disc that indicates that there is actually light missing. Before that, there were astrophysical observations that strongly indicated that there is a black hole, for example QSOs, that is a bright combination of accretion disc and jet, that is powered by a black hole, or one paper I really liked tried to argue that there has to be a black hole in Cygni X1 because otherwise we would see the missing matter crashing into something. (Cygni X1 is a binary of a star and something very bright and the star is loosing mass.)

Because they're very small, very dark, and very distant, compared to visible objects like planets in our solar system, stars in our galaxy, and other galaxies.