That's because of how the attention focus mechanics of LoL are designed. I'm not talking spectator mode, but the whole game design.
I planned to finish writing a small article on that yesterday, but it didn't pan out, hopefully today, and it'd go up on Reign of Gaming with some luck by the weekend.
Edit: Here's a super brief from a comment I made on another thread yesterday:
Darius does "steal" the spotlight, but that's not an issue with Darius as much as where the game's attention is focused:
Kills give 100% gold, whereas assists give less gold, and must be shared with others.
Kills get announced by the announcer.
Kill Streaks and multi kills get announced by the announcer.
End game screen, you have one line for assists, but 3 for kills (#, streak, multi).
Ranked game statistics has k/d as one ratio, and k+a/d as another.
Darius is designed to be in the spotlight. Darius is designed to get kills - often these are kills that'd be there anyway, which is why I don't think he's OP - he usually redistributes the kills rather than generate them over other top of the line tanky-dps champions.
Now, is that a problem with Darius, his effect on spotlight? I think it is, but not with Darius, but with the spotlight and reward cycle mechanics. These are also a part of the reason less people play supports - sure, internally, or once you play with people who understand, you can have those "Yes! I did amazingly there, CCing their team, saving two allies, as Janna, and making that a 3 for 0 exchange!" but you don't get the game-mechanical appreciation.
Changing those things, to people would care less about kills per person and more about team success as a whole (this IS a team game after all), would benefit many things in the game. Darius' effect on the game is a lot on the perception side. Now, I'm not saying it's perceived only, but the game mechanics do a lot to focus attention and control the perception, and they could use some tweaking.
Darius is also on the overt side of the power spectrum, as opposed to many CC-bots or supports - they don't necessarily have less power, but their power is diffuse/less noticed by the game client (all those kill announcements, remember?). Darius does a lot of damage. I think the nature he gets all the attention in game is the cause of many of the OP claims - people think he's OP, then they go forth to find what in his kit makes him OP