I haven't watched the interview above yet so this could be total BS, but I've felt for some time that Hilary's view on music is similar to something Cheryl said around her Love Made Me Do It era - that she wasn't going to continue / didn't see a point in doing it if it wasn't successful. I think after giving it a second go with RCA and the Breathe In Breathe Out album and it not really working out for her, she sort of decided there's no reason to put out the bait if the fish aren't biting, so to speak.
I think it's more about diminishing returns for her. She views both acting and singing as jobs somewhat, and there's no reason to return to a job where you're not necessarily succeeding, no matter how much people liked you working there.