Technically, I am not buying the "there is no reason" either most of the time. It is probably any number of common reasons like red flags (filled with buzzwords and marketing speak, weak/iterative product, single founder with no history of success) or maybe it is just a bunch of more minor or subtle things. Maybe they don't think there is a huge future for X even though it is popular now; they think Y is on the way up or there are just 10 other applications doing something like X which are more compelling or have a team with a better pedigree than you.
Maybe sometimes it is fairly easy to give a reason, but then it will be expected for everyone and they get a lot of applications now. It would be hugely time consuming to give a detailed evaluation and maybe sometimes too it would be giving away some secrets they don't want to.