Let's look why Ahch-To (which was also not in official map) was a better option than Dagobah:
Ahch-To had historic importance for Jedi. It hosted the first Jedi temple. It also had an ancient Jedi tree which contained first Jedi books.
Ahch-To had focal point of Force balance thing with concentration of both light side of the Force as well as dark side of the Force.
Ahch-To seemed to have more diversity of life. If not, it at least had intelligent caretakers who were devoted to Jedi Order.
Why didn't Yoda choose to go to the Ahch-To?