The main reason is that allowing transfers right now would cause problems. You can see their official stance on transfers in the FAQ page:
"We have not built, and do not plan to build, the capability to move individual characters from platform to platform. We were only able move characters in bulk for console once on June 1st 2015 because the database tables we copied to were brand new (empty). Now that the database tables are filled with other data, the techniques we used before no longer work."