You needn't explain to anyone why their post wasn't upvoted... Anyone who thinks they're entitled to an upvote from anyone else just because they wrote about a certain topic, or used a certain tag, probably doesn't deserve one, and if they're directly calling out @steemstem for not upvoting their post, they may even deserve a flag... Just my opinion.
Either you didn't see it, or it didn't meet your standards. There are things people need to understand about Steemit in general, and that's not the responsibility of a niche community.
What I will do to help:
I put forth an idea for a multi-tier STEM themed contest that would run on a bi-weekly basis. I will start working out the details, and get that started up, as a way to not only get people creating STEM content, but to improve their work through competition, and encourage engagement.
Contests are nice to trigger engagement. I am however not so sure that contests will maintain engagement on the long run, from my own experience. However, I have no idea about what you want to do in details. Maybe I am wrong. Hopefully I am wrong... I am more than happy to support anything that anyone will try. Anything that helps, is welcome. Anything :)
Therefore, if you want steemstem to support the contest, feel free to contact me on discord (before starting the contest) to see how we can help you :)