In Agricola, why are Minor Improvements not separated by number of players whereas Occupations are?
The purpose of the marking is to help the players to remove the cards from the deck before playing which do not fit the player count.

Specifically, many of the occupations refer to spaces on the board that are not present in lower player counts, such as "Travelling Players", which is only present with 4 or more players, and thus all of its associated occupations are only relevant with 4 or more players.  The Occupation cards are marked based on number of players so that you can remove the ones that are not valid for your player count.  

However, Minor Improvements are valid at all player counts, so you do not need to go through and remove any of them.  

Uwe Rosenberg could have designed the Minor Improvements such that you removed some of them at lower player counts (in which case those would be marked), but he did not choose to do so.