Deep Dungeon is more of a classic 100 floor tower challenges we see in RPGs where players either reach the top alone or with a group of controlled NPCs.

The Separate leveling is a means to keep that ability for the player to climb to the top alone for that Solo challenge but will require them to put more effort into leveling their characters. In a group they can easily reach the top but groups are not put on the Leaderboard as that place is for players who solo the Deep Dungeons.