Beast Boy has been digitized, and can be found in the book Teen Titans by Geoff Johns Book Two:
https://www.comixology.com/Teen-Titans-by-Geoff-Johns-Book-Two/digital-comic/633479
https://www.hoopladigital.com/title/12047226
I have a thread here that serves as a directory to where digitized comics that aren’t on DCUI can be found:
But yes, I agree that Beast Boy and other series like it that have only been released in digital collections should be made available on DCUI. Until that happens, though, hoopla serves as a decent alternative.