It was a major problem before BP consumption on Expert raids was reduced from 3 to 2. There weren't enough people joining Raids so many ended up timing out.
It's not really a willingness of stronger players to join. I can solo Raids so it matters little to me whether others in the Raid can pull their weight, and in fact I get PP and MVP bonuses if my team on Auto ends up doing the lion's share of the work, so I try to join full HP Raids with few players whenever I can.
The actual problem is that low-level players have low-level friends due to the way the matching system works, and therefore low-level players have to rely on swarms to get anything done. Any factor that reduces the size of those swarms directly affects their ability to clear Raid content. If I'm a low-level player, and a low-level friend calls for help on his solo boss encounter, I'm going to be hesitant to join if I don't think others besides me will join. In this way the problem quickly snowballs--people stop joining because they don't think others will join, and only join raid bosses where a critical mass of players have already joined (often caused by a high-level player starting the fight or joining and doing significant damage, causing the circling opportunists to swoop in).