There are a couple of reasons behind this, anonymous.
First and foremost, there are cultural differences at play. In comparison to Sinnohans, Alolans are blunter and gifted with a slightly darker sense of humor. Perhaps it has something to do with the capricious and often violent nature of their island guardians or perhaps it has to do with the fact that every island except Poni features a major site for the dead, but death is just a part of life to Alolans. They celebrate it as being an honorable or otherwise inevitable state of being, the final end to what had hopefully been a fruitful and adventurous life. As such, they talk about death in frank terms; it’s both important to talk about and a subject that one can make light of. So when you see an Alolan entry refer to death, it’s not meant to be morbid but rather a statement that death can and will happen to even the best of us. This is in stark contrast with the Sinnohan viewpoint of death, as Sinnohans have a tendency to avoid the concept altogether, likely due to its own (rather negative) folklore on the subject. Likewise, the Alolan pokédex may be gorier than any other regional index, simply because Alolans in general are a lot more open about discussing topics that are considered taboo or otherwise impolite to cover in other cultures. Even Unovans tend to be rather prudish concerning violence in comparison with Alolans—or at least, Unovans are when it comes to showing violence to their ten-year-old trainers.
In short, a lot of it has to do with the fact that the writers of these entries tend to write according to their culture, and the Alolan dex, while it had help from a Kantonian, was primarily edited by an Alolan.
The second reason, of course, is the fact that it was also edited by the ghost-type rotom, but it’s important to note that not all of the morbidity is because of the rotom by itself.
The Pokédex According to Bill 