As a publisher, I'm always looking for new games to publish. I'm fortunate to receive a lot of games submissions on a more or less constant basis. Quite a lot of these are types of games I'm not interested in (mass market, trivia, sports), and so can be instantly discounted. Some sound interesting, but have so many components that I have to rule them out on cost. Lots sound interesting at first blush and so I ask for a prototype to play with my playtesting team.
Some of those I try aren't very good when you actually play it and can be quickly discounted. Occasionally, you get an awesome one which is pretty much ready to go (Sumeria was one of these). The vast majority however are 'good'. Which is a bad thing.
There are tens of thousands of board games out there (BoardGameGeek has over 50,000 in its database) and hundreds more get released every year. If you're a big company like Fantasy Flight or Games Workshop, you've got the marketing budget and market presence that means your games will sell well. If, however you're a little guy like me with a tiny marketing budget and very little market presence your games will have to fight tooth-and-nail to sell themselves. For comparison, the third edition of Space Hulk was released two weeks ago. It has 761 ratings on BGG and 1554 people listed as owning it. Sumeria has been out fourteen weeks and has 100 ratings and 116 owners. Very few people rush down to their local games store to buy the latest Reiver Games release when it arrives on the shelves. Instead they might hear about it, add it to their wishlist or want to play list. Maybe wait for a friend to get it so they can try it out, or hope to find it at a convention where they can try it out. Of the much smaller number of people who are interested and who find a copy to play, only people who really like the game will tend to buy it.
If I publish a game because I think it's good, the chances are most of the few people who play it will also think it's good. Nobody rushes out to by a good game. They rush out to buy a great game. The internal conversation goes: ' I know Bob's got Zombie Ninja Pirates in Space, but it's frickin' awesome, I need my own copy', not: 'I know Bob's got Watching Pastel Paint Dry, but it's a pretty good game, I need my own copy'.
I need games that at least I think are great, in the hope that I can find several thousand other people who think the game is great enough to warrant buying.
Still, it's very hard to go back to a designer and reject their game because it's good. There's nothing really wrong with it, it's solid, it works. But it's not great. It doesn't help that great is subjective.