You mean beside the reasons i already gave?
Because it would alienate potential buyers. har har drm good har har, customers bad. This is an attitude from the past that gets you nowhere.
I think that if you really look around, you will see that drm does no one any favors. The fact that these developers treat us as potential customers instead of potential pirates really goes a long way.
I don't doubt that the game will be pirated. It's a fact of life nowadays. Is that good? no of course not. Is it the end of the world? depending on how you handle it, definitely not.
If people never planned on buying, drm wouldn't convince them otherwise. If people are on the fence about it, having no drm might sway them to give it a try. And even the people who pirate the game, still do free advertising for you