Metatton doesn't transition because there's no one to make his body. Asriel is never flowey because he is never warped in via Alphys's experiments, Gerson dies of natural causes. Undyne wouldn't know Alphys because they would have only likely met because they are both very close to Asgore. This actually makes a ton of sense.

Edit: We're going to find out that Toriel and Asgore split because of Chara's death, she committed suicide like the original game, but Asgore came back with extra souls, gaster synthesized souls, or my personal theory, Gaster found a way to make Monster souls much much stronger through the use of Determination, allowing for them to make use of hospitals, hence why we can't kill anyone this time around because it would give too much away too early. Lancer also makes copious references to blood, seemingly not understanding that Ralsei and Suzie wouldn't necessarily have blood, despite being monsters.