Why is Weatherlight not included in the format?

Weatherlight would indeed complete the block, and we actually considered including the set in the format; however, the drawbacks were a bit too much.

First of all, the set was directed completely differently in terms of design and lore (big thing as flavor is pretty important in old formats).

Despite being part of the Mirage block, Weatherlight has not a lot to do with Jamuraa and the three nations and it embodies the shift in design philosophy that would come with tempest. This is because originally Mirage and Visions were just two sets created from a set idea "Menagerie". Weatherlight was added later to have 3 sets blocks.

There is a lot of graveyard interaction in the set, and with Bazaar of Baghdad (and Necro!) being allowed, reanimator is already pretty good. And of course, cards like Doomsday or Buried Alive would have to be banned.

There are also too many “draw a card” effects, which could create too much consistency with the Mirage tutors being legal.

Overall, Weatherlight feels too close to the Tempest block compared to the other sets in the Mirage block, and even with a banlist of 5 or 6 cards (Abeyance, Serenity, Buried Alive, Doomsday, Rogue Elephant), it seemed like OS strategies would need to change a bit too much (and we want them to remain relevant), while other archetypes could become too strong, in addition the flavor and feel is a bit different.
Plus, historically, Visions is the set released alongside Shandalar and Fifth Edition, and when Pro Tour Paris took place.