It's because the frequency a mission gets played depends on how far down the list it is. If you're in the top spot you'll get multiple plays per day. If you're way down you'll get maybe a couple per week if you're lucky.
Maybe this is what people don't realize, by dominating the list the grinders already are harming the story missions. The amount of plays they get is dramatically reduced since they're buried under so many other missions, because even though some people might be interested in them, many simply don't realize they're there. A lot of people will come into the Foundry and play Battleship Royal Rumble and think that's all there is to the Foundry.
When you look at missions that were made in less than 5 hours, and see they're all dominating the top of the list, resulting in your mission getting half as many plays, or less, than before, it makes you question how much effort you should put into making missions. I know I personally don't feel as motivated knowing that all the community at large wants is grind missions anyway, and anyone who does happen to be interested in story will have a difficult time finding my missions anyway.
Foundry authors have had a lot of complaints about the top rated list even prior to the grinders, but now those complaints are magnified even more.
Can I just point out that you made a comment, which kind of shows that the problem doesn't lie with the larger part of the community; "knowing that all the community at large wants is grind missions anyway". At the end of the day, if it's what the majority of players want, the game would suffer more to remove them than to leave them in.
I can understand where you're coming from, I've made a few foundry missions myself, none of them have astronomical play counts, but at the end of the day I didn't make them for that. I made them to tell a story I enjoyed, the people I care about have all played them and given their feedback and beyond that anything else is a bonus.
Also, the in game menu system is really the thing at fault here, not the players who don't have all day to play an hour long mission filled with text. There should be a better interface for finding missions in game. I know that personally I don't ever browse the list of available foundry missions because there are simply too many to look through. I use the foundry forums section to check out info about Foundry missions I like the sound of then search for it directly in game.
Like I say, I can understand that it's upsetting to not get a thousand reviews on your missions, but really if that's what you're making them for then you're in the wrong place. Gamers are notorious for using the quickest/easiest method to get to what they want, not for sitting playing well constructed/written missions and giving glorious feedback.
Join date is wrong, I've actually been around since Beta.
99 little bugs in the code, 99 little bugs.
Take one down, patch it around.
127 little bugs in the code...