itch.io

The mythical status had traditionally been for the Pokemon being tied to timed event releases. Darkrai was released in a event Cresselia was simply in the main game. It is quite effective because before the event release we got a taste of Darkrai and the nightmares they create before the proper confrontation. It's why Darkrai imo is one of the most memorable and best Pokémon period because they genuinely had build up for him and sold him in an interesting way. 

As for Heatran, Chainsaw didn't forget. I'm the one who pitched splitting the update due to scale and I also pitched trying to rebalance the split. If they did the sub/mythical - Arceus then main legends + Arceus split they planned originally, one update would be a lot larger then the other and it would be getting 7/4 legendary events wise and a 9/4 split if we counted the legends involved. In the end, even though I original pitched swapping the lake trio, Heatran has been moved over to 4.2 for a more balanced content split which is perfectly fine imo especially when you realize Heatran is the most isolated of the subs/mythicals. Heatran has the human pairing planned out and is the Futa of this region (literal 50/50 gender split if you wanna know why) so plans are in place, though feel free to air it as it could help with planning Heatran out.