Ash has had so many female companions that the ones in the middle are going to wind up forgotten. Already Iris seemed to be the least popular of all the girls, but a lot of people are starting to forget about Dawn as well even though she had a fanbase. When Serena leaves her character will wind up sandwiched in the middle too.

It's called, "middle child syndrome," where things in the middle tend to be forgotten. I think the only reason May escapes this is because she was the first new main girl of the series, so she'll always be remembered for that reason. The others really don't have this luxury especially since every girl leaves after a single gen.