Why didn't Yoda go to Ahch-To instead of Dagobah?

Question

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?

Answer 1

There are several reasons:

1. Yoda likely didn't know where Ahch-To was, until he became a Force Ghost. Luke only found out after he had access to both Imperial (thanks to R2D2's hacking) and New Republic archives after the Rebellion won and the New Republic was established. And even then, finding it was a big difficulty for Leia, who had access to same info. R2D2 had to get the info from Imperial databanks, which Yoda didn't have access to.

2. Alternately, even if he knew from Jedi databanks (we aren't sure if that was how he was able to visit Luke as a Force Ghost in TLJ, or from his post-Force-Ghost knowledge), then it would have been an awful place to hide from Emperor Palpatine, who managed to get full access to those same Jedi Databanks after Order 66 destroyed the Jedi.

3. Yoda was acting upon the will of the Force (as per Matthew Stover's Episode III novelization) - he was expecting Luke to be brought to him for training when the Force did it; as such, presumably the Force told Yoda to go to Dagobah.

   “We should split them up,” Obi-Wan said. “Even if the Sith find one, the other may survive. I can take the boy, Master Yoda, and you take the girl. We can hide them away, keep them safe—train them as Anakin should have been trained—”
   “No.” The ancient Master lowered his head again, closing his eyes, resting his chin on his hands that were folded over the head of his stick. Obi-Wan looked uncertain. “But how are they to learn the self-discipline a Jedi needs? How are they to master skills of the Force?” “Jedi training, the sole source of self-discipline is not. When right is the time for skills to be taught, to us the living Force will bring them. Until then, wait we will, and watch, and learn.”

4. Dagobah is a swamp. If you use our Earth for a model, that's about the most life-density-possible ecology except for maybe a tropical forest. Definitely far superior to Ahch-To ecology.

5. Yoda already knew of Dagobah and the Dark Side tree.

   • Disney Canon

     In Clone Wars, (a) Qui-Gon's Force Ghost told Yoda about the tree; (b) R2-D2 went with Yoda to Dagobah.

   • EU canon

     Yoda was intimately familiar with Dagobah from prior exploits. That was where he met and defeated a Dark Jedi.

Answer 2

I'd like to add that Dagobah is a planet that is rich in the dark side of the force. The planet helped shroud his light side presence from the Emperor. This took a toll on Yoda's life considering that the light side of the force helps its user rejuvenate while the dark side is more like gaining power at the expense of one's health and well being (Darth Nihilus and Darth Sidious are great examples). Therefore, being surrounded by the dark side perhaps worsened Yoda's state of health.