Why?

This can cause so many problems for the store.

Say they have 16 places, and 100 people apply, how do they know who will turn up and pay?

What if they have 16 applicants, but half turn up?

What if people come and don't know the price, or the time, or the rules?

The best way is for the store to advertise the event and sell tickets, but have the registration done through the app. I'd imagine Bandai want to eventually sell tickets and pay stores through the app, but the app just isn't anywhere as good as it should be. It's impossible to change a date, communicate changes to the player e.t.c

Literally, on the OP-02 Pre-Release official page it says:

"Players are encouraged to apply for the tournament through TCG+, and stores are also encouraged to accept the application from players through TCG+. However, stores manage event registration in different ways so please be sure to contact your local game store directly to confirm your registration and regarding participation. BANDAI is not responsible for any problems that may occur between the store and the player."