Creating a (secure) market is no easy task. Lots of planning and lots of development. DHL already has the secure system, if they simply lifted the invite-only requirment for buyers they could be the top market overnight RIGHT NOW.
Any guesses on why they don't do this? Yes there are many advantages to an invite-only market but I doubt (tens of) millions of dollars in additional revenue compares to those advantages.
edit: hell they could even fork dhl code and create a seperate market that isnt invite only
Because opening the market now would draw attention to it and fuck it up as well. Everyone take a brief hiatus from onion activities and leave private markets be until this all passes. In the mean time, a market is being developed.