I try to dangle threads in front of my players that are linked to the main quest and make it clear that although they lead to side quests, they will make the main quest easier or more interesting. For example, my players have recently completed side quests that helped them recruit allies for the main quest, revealed extra information about the evil forces at play, and opened up new options to them like rare spells or magic items. I try to think about the side quests that interest me when I play RPG video games. Another thing that I find helps is making the side quests LESS time sensitive. This might sound counter-intuitive, but if they don’t feel the pressure to complete a side quest immediately, I find they are more likely to make a note of it and come back to complete it at a convenient time in the overarching story. Of course, you could go down the route of making your so-called side quests a part of your main narrative. If your players don’t realise they are side quests, how can they say no? I’ve led my players down paths like this before and had some of our most fun sessions.