Except Amash has been principled throughout his entire term, not just this last cycle after deciding not to run again. Of course, it's hard to look at every vote/comment he has made over the years since that's too much, but just as one example, he has been consistently very vocal with his criticism of house leaders from both parties that don't allow for the proper legislative process.
If he's so principled, why didn't he propose this bill during a time where he could actually see it walked through the different committees and maybe have a chance at getting to the Senate?
I don't know, there's a lot of possibilities. I doubt it's just because he's been sitting around doing nothing this whole time (but if you think so feel free to say why you think that). It seems more likely to me he was busy with other stuff, eg like his bill earlier this year to end qualified immunity. At the end of the day it's all about prioritizing, and those decisions can become pretty complex I imagine.
You could always try asking him yourself, he seems pretty responsive to people on twitter, and here [1] he answered a somewhat related question.
