The projects are linked to the account of the user who created them, who we define as the project owner. The owner has maximum permissions over the project, including removing it permanently. Also, if a user cancels the account, all the owned projects are automatically destroyed.
It is possible, however, to transfer the ownership of a project from one user account to another. This is convenient when the owner is not involved in the project anymore, or their account will be removed for whatever reason, but the rest of the team wants to continue working on the project.
You can transfer the ownership of the project from the user permissions dialog. Go to the Administration -> Users & permissions section of your project, and double click on the user who will be the new owner (also accessible by clicking on the ellipsis icon). Below the user picture and name, there is a button to init the transfer: