Are they optional? Do they provide a power boost that is necessary for the higher levels? From what I remember the books presented them as a non-optional part of character advancement.
Page 27 of PHB "If you just reached 11th or 21st level, you have some exciting decisions to make—you can choose a paragon path or an epic destiny…"
Not "you choose a paragon path or an epic destiny…" or "you must choose a paragon path or an epic destiny…"
Now, not sure why anyone doesn't pick as they're free stuff. But there's no requirement to must take them. And DMs in 4e, unlike the common refrain, have just as much power as they do in every other edition to put their foot down and say, "Hey, everyone does this." — you want everyone to take Destined Scion to keep things simple and more martially for the martials? You can do that. 4e literally just suggests you explain your reasoning and try to get buy-in, so people don't find out too late to make decisions or get attached to something you'd rather them not take and then have your game blow up.
Page 189, DMG: "You have the authority to do whatever you want with the game, but your efforts won’t help if you have no group."