Cause you haven't been invited yet. If you actively play the global server MLBB for certain period of time (active as in following all events, playing arcade games, and ranked) then you can get an Advance Server Invite on your mailbox. Once you get it, open the mail, tap the link (this is crucial since the link won't be unlocked unless you open it from that link, and you also have to install TapTap before the link, which you already did rn), and you will be directed to the MLBB Beta here, and the download links will be unlocked permanently.
