May went to Johto back in the end of AG and briefly went to Sinnoh in DP! Keep in mind that Ash's friends are not static, they are living their own lives, going on their own journeys off-screen. ^^
I am sad they didn't meet when he went to their regions, but there absolutely must be chances for that in the future!! Others have mentioned that in previous series, it usually took about around 40 episodes before old characters reappear.
Misty and Brock recently did appear in silhouette form!
The writers and animators, from what we see, do love the old charactera (Iwane Masaaki-san and Shinohara Takashi-san are always drawing older characters on their respective twitter, expressing their wishes to see them again, Iwane-san drew and coloured and animated Shinji/Paul in SuMo style and drew a beautiful Iris in her Champion dress wishing to see her return in it in the anime...), but the executives only care about promoting new games, typically...
There recently was Lt. Surge telling everyone about Ash, so there is continuity yet! I really do hope that with this series taking such a different approach from past ones, there'll be more scenes with older characters. We didn't get that in the second anniversary movie, despite the director considering it, so this series would be best to showcase old friends and rivals.