It's because of how ESEA's rules are designed to promote roster longevity, something which isn't really respected given how many teams die off and reform each offseason. It now takes four seasons before a team can be part of the Pro League in CS (with hijack rules to prevent most teams from skipping part of that process), and ESEA won't let up on that by doing placement tournaments (the closest thing is the two promotion/relegation matches for the Pro League, something new this year).
We are lucky that we don't have the rigid structure of Pro League and that teams can still be manually placed in Invite by an admin, mainly because we don't have the size necessary to make something like that happen, but lower divisions don't have that privilege and must follow ESEA rules.
It's not something that the TF2 admins can really be blamed for since it seems they're given a tight leash when it comes to running the league the ESEA way. (Remember when they almost got fired for turning a blind eye to Open finals being played off-client?)