IMO the one of the reasons why Selena is not getting a third single is because she cancelled the tour.
HWR is probably mad at her for that or maybe
she's the one that doesn't want another single...
Demi is getting a third single (and probably a fourth one) because her tour will start in a month and the era is still going for her so...
I hope they prove me wrong and we get a third one but at this point, i doubt it :/
But then again i'm not an "expert in marketing" and this is my opinion so... don't drag me