It's simple really, you get tomestones from those dungeons which can be used to get the gear you need. Leveling dungeons have 0 means of getting what you need if you get nothing, that's why they added it, so when you are leveling, you have a better opportunity of updating your gear.

Lv 50/60/70 dungeons don't have this issue because of said tomestones which allow you to purchase better gear.