After Luke Skywalker had failed his Jedi Academy and went into exile, he hid so well that it took years for the Resistance to find him, requiring a special map leading to his location. His island on the planet Ahch-To was one of the most difficult places to find in the Star Wars galaxy, and for good reason, as the Jedi had intentionally kept it hidden for thousands of years due to its special connection to the Force.
As seen in Star Wars: The Last Jedi, Luke's island houses a unique Jedi tree and ancient Jedi texts -- so clearly the planet was important to the Order in some way. And while it wasn't explained explicitly in the movie, it's since been revealed why Luke chose Ahch-To. As not only was it a sacred place for the Force, it was the actual birthplace of the Jedi Order itself.
The Origins of the Jedi Order
As stated in The Last Jedi Visual Dictionary, the Jedi Order was founded on Ahch-To around the year 25,000 BBY (25,000 years before Star Wars: A New Hope). Its founder was known as the Prime Jedi, a figure who was depicted as a mosaic near Luke's home; although their name and race are unknown. Nevertheless, it's said that years passed while the Jedi founders remained completely cut off from outside visitors, just like Luke.
When Han Solo talks about Luke in Star Wars: The Force Awakens, he says, "People who know him best think he went looking for the first Jedi Temple." That comment suggests Ahch-To is that place, which would certainly make the most sense. Although, the Star Wars: Complete Locations canon reference book suggests the first Jedi Temple may have been on Jedha from Rogue One or the city planet of Coruscant. But, Ahch-To still seems to be the most likely location.
Ahch-To Was Hidden By the Jedi
As space travel in the Star Wars galaxy began to expand, more and more people started discovering Ahch-To. And with more people came the chance of conflict and ruin. Fights against the Jedi religion eventually broke out, to the point where the native species (known as the Lanais) were forced to move away. And so, the Jedi decided to spread out across the galaxy and leave Ahch-To in peace.
But with how sacred Ahch-To had become, it was a symbol for the Jedi Order that was too precious to completely ignore. And so, many of the original Jedi texts were kept there for safekeeping, and the Lanais became the caretakers for the temple. The Jedi soon left and kept the location of Ahch-To as hidden as possible, so much so that the Empire failed to find it during Order 66.
There were several reasons why Luke chose Ahch-To as his home in The Last Jedi. He needed answers for whether the Order was worth continuing, so what better place to decide its fate than where it all began? Plus it had the added bonus of being a secure world cut off from most charted maps of the galaxy, something that was becoming incredibly rare over the years.