The decission for a Pokémon coming back or not is not made by coders or developers, not even by GameFreak, so they just code everything and wait for orders.
It is TPC who decides who to shill, who to ignore, who to keep waiting for a later return, all based on merchandise release schedules.
People need to understand that the vast majority of pokémon's earnings, like with every children-focused franchise, comes from the merchandise, from selling rights for toys/plushies/clothes/cereal/bags/towels/pencils/cases/etc.
To maximize earnings, they plan who to shill, at what year, and time of the year based on their business deals with each merch company, with their TCG releases, with their movie releases, anime episodes, etc. to market their shilled mons in the most efficient way and sell as much as possible. Organizing those schedules is the entire job of TPC and the reason it was created after gen II released. (Because GF could not keep up with the high merch demand alone and Masuda was having anxiety attacks as Director. They were at their limit).
So now that Home is a key part of their business strategy, because it links the main games to Pokémon GO's stellar success, they decided it was a good idea to artificially make some mons missing, to make their big return more hype in the coming years, while focusing on the new additions more today.
Notice how it's mostly gen IV mons missing now? Expect Twitterfags posting shit like "finally my boy Buizel returns! After so many years! #DiamondandPearlRemakes #Hype"
It's all planned.
