My question is related to black hole image released in April. As far as I understand idea of EHT, it joins observations from multiple locations to work like one telescope with radius that is equal to the distance between the farthest telescopes in array. Moreover Earth atmosphere affects observation, e.g. by introducing delays. Would it beneficial to add Hubble Space Telescope to EHT and if yes, why HST was not included in observation array?
No, it would not, because it operates in the visible spectrum and the EHT is an array of radio telescopes. For the "very long baseline interferometry" technique to work, all the telescopes have to be operating at the same wavelength, because combining the signals involves measuring exactly how well the peaks and troughs of the radio waves from the different telescopes line up.
You can do VLBI in the visible spectrum, but you have to match up the waves even more precisely since light has shorter wavelengths than radio waves. The EHT collected all the data first, and then spent a lot of time combining it by computer, but that required using very precise clocks to sync the data. We don't have clocks precise enough to do that for light, so a direct optical connection is required between the multiple telescopes. So there isn't a good way to do a planetwide VLBI optical telescope yet.
Addendum inspired by the comments, especially from TazAstroSpacial: To get the same resolution with optical light, you can use a much smaller telescope array. I didn't mention that fact because I was thinking about keeping the size of the array the same and getting better resolution. But in any case the problem is that the challenges of doing the interferometry at smaller wavelengths more than make up for the advantages of needing a smaller aperture. At least at the current state of the art.