First off, Portal 2 was eight years ago. That's a long time and mainstream consumer expectations for what a video game should offer have changed a lot over those eight years.
Secondly, I know plenty fans of HL/HL2 who found the tone/direction of Portal 2 distressing because it was too self-aware or whatever. I don't really agree with that, but it's a point of view that's been circulating around for years. I think a lot of fans have come to terms with HL3 never being released. Continuing to not release HL3 has little apparent risk. Less risk than releasing it and making something a bunch of people will either hate or ignore. So I don't think they will.
Secondly, I know plenty fans of HL/HL2 who found the tone/direction of Portal 2 distressing because it was too self-aware or whatever. I don't really agree with that, but it's a point of view that's been circulating around for years. I think a lot of fans have come to terms with HL3 never being released. Continuing to not release HL3 has little apparent risk. Less risk than releasing it and making something a bunch of people will either hate or ignore. So I don't think they will.