TL;DR: In-universe, we don't know, but it may be a leftover from the Rakata Empire. Out-of-universe, Lucas liked the sound of it.
In-universe, we have no explanation for why some 15 planets or so all end in "ooine". Most of the said planets are located in the Outer Rim territories, and many currently fall under Hutt control. However, some of these names have existed for millenia, predating even the Hutts.
The best possible guess at the moment is that the suffix had some meaning to the Rakata - an ancient empire which controlled much of the galaxy eons ago, and presumably named many of the planets in their empire.
Out of universe, Lucas first invented the suffix when attempting to name Luke's home planet. The name "Tatooine" is a homonym of "Tataouine", which is the capital of Southern Tunisia where the Tatooine scenes were filmed. Lucas has stated in interviews that he changed existing non-English words to give them an alien feel.
The only two planets given this suffix in the original trilogy were Tatooine and Dantooine, although early Legends canon introduced the world Klatooine as the homeworld of Jabba's Klatoo guardsmen. Afterwards, the suffix became popular whenever an EU author needed to make up an Outer Rim planet.