Lillie- Current series pokegirl and another popuar character
Brock- Longest travelling companion and most popular companion
Serena- Second most popular companion and ash's love interest
n
It's probably more that Lillie is the current girl and Serena was the previous gen girl, not really to do with popularity. And Brock is just Brock. Otherwise I'm sure May, Dawn, etc. would have gotten a sequence too. They were just focusing on the recent companions.