One of the main purposes of the Atlantis portal was to help players asking for another chance to get HOTM cards they missed, this is why previous HOTMs are cycled in an out. It doesnt take away from the chance of a bonus summon for a current hero of the month.
The 5 star Atlantis/S2 heroes are also cycled in and out which is why Misandra wasnt present last time but this time Tarlak is not.
Mok-ar is a newer Atlantis hero, he was only present in this atlantis event and the one before.
There’s probably no new heroes for the atlantis portal because new heroes were released for the seasonal summon instead.
You don’t have to worry that they’re going to start ignoring the Atlantis portal. Info on new atlantis legenary heores in the works are available thanks to beta testers. Three legendaries for the Sakura flower family are in the works: dark, holy, & fire. Two legendaries for the Atlantis family are also being tested: holy & nature