Sometimes, it's because they can't play LNs, and it is frustrating.
Sometimes, it's because they can't understand how LNs are used.
Sometimes, it's because they just don't enjoy LN patterns for some other reason.
Games like LR2 and Stepmania don't use many LNs, so players who came from those games are not used to LNs. Since they are not used to LNs, they might get frustrated when they can't play something with LNs, or they just might not enjoy LNs even if they can play them well, because they are so used to patterns that do not use LNs. Of course, new players might also not like LNs because they require a different kind of skill from playing regular notes.
Personally, I think there are not enough easy LN maps, so new players won't have any real LN skill until they begin playing harder maps. When I played O2Jam, for example, I found out that around level 30, maps begin using LNs a LOT more often than easier maps, and it creates a skill wall where you need to practice for a long time before you become good enough to start playing maps above that difficulty level.
