It's because MastaC built the deep market and owns it

Does a shop have to be in the marketplace or can it be next to your house?

You may start a shop wherever you want. However, may not be seen much, depending on its location, and it might be more vulnerable to griefing. Just as a reminder, the only place that you may not make a shop is in any one of the spawn residential lots. Those lots are strictly for houses.

Since this thread is old, I'm going to lock it. If you have any questions, please ask them in the Official Spawn Marketplace thread.