Because Mannion isn't the future.
I haven't seen any of our QB's do anything that made me go "Wow."... And that's okay, but people have a tendency to make these guys more than what they are.
Keenum is a game manager, he's not someone that is going to carry the offense week in and week out, but he'll minimize mistakes (for the most part) and generally speaking wont lose you games. People are trying to make him into some elite QB, and he's just not that. I'm comfortable with him until Goff gets going, but long term? No.
Goff is a rookie with good upside.. Just how high his ceiling is we don't yet know, but it's the highest of any of the other three. His footwork looks good and should get better, the accuracy and release is there, along with touch and arm strength. He needs to work on reading defenses and slowing the game down, but that's all going to come as he gets adjusted to the NFL, I don't expect anything else from a rookie at this point. Given we have Keenum we can be more patient with Goff, which is to our benefit. At this point Goff is not the best QB on our roster, but he is quickly closing the gap between him and Keenum.
Mannion is a backup, and that's about it. He's got good armstrength, but I'm not seeing it right now. He played well against the Cowboys, but his footwork isn't as good as Goff's, his accuracy isn't as good, he's got a lot more ground to makeup to give us a good chance of winning. His future is likely Goff's backup after Keenum leaves at the end of the year, but nothing more. He's not the future of this team, and we shouldn't pretend like he is. He's the 3rd best QB right now.
If Goff isn't the future QB of this team then we're in trouble because that means we don't have one on our roster.