In two-player and solo Caverna, why is Round 9 with the Exploration action skipped?
TL;DR: Ultimately it's a matter of game balance

The Exploration action card is removed as a result of two factors:

1. The 2 player game is shortened by one round (and one harvest)


  In a 2-player game, remove one of the Harvest markers with a green leaf from the game. Then distribute the remaining 6 markers among the Round spaces 6 to 12 leaving space 9 empty.


Having one fewer round makes the two player game harder and reduces the amount of time the players have to implement their strategies and see the returns. The reason for this is because you have less competition over board spaces and so the main constraint/obstacle needs to be time instead of the other players

2. Because one of the action cards needs to be cut in order to remove a Round, one had to be chosen.

Exploration makes the most sense
In a game where time is the main obstacle instead of the other players, it's easier to play the actions of your strategy. Therefore, action spaces that provide a higher quantity of resources or a higher versatility of resources are more important. In this situation, the ability to embark on a level 4 expedition is both, and that heavily imbalances the overall strategy towards weapons.

This is problematic because Caverna aims to make all paths to victory equally feasible