A: Because in the solo game, you can only start a “floating island” with a Beacon card. Though this is not explicitly stated (we should probably tweak the rules on the next printing to do so) it can be inferred via the following:

Per the Basics of Island Building rules common to both ways of playing, “A Beacon can establish its own new island whenever needed.” However, only the multiplayer rules state that any terrain card can start a floating island.

This is because, in the multiplayer game, there is no discard option: you are required to play a card on your turn. It’s possible that a player might have only non-Beacon terrain cards that cannot be played onto an existing island. For this reason, in the multiplayer games, you can start floating islands with any non-Beacon terrain card that cannot be played onto existing terrain.

In the solo game, the whole challenge is to be able to play the deck of terrain cards into a single connected group of islands in the order they come up, and if something besides a Beacon is unplayable, it goes in the discard pile, and you’ll have to retrieve it by using up one of your Ships if you want to use it.

See also: Do any of the multiplayer rules carry over to the solo version?