Game starts with a difficulty spread from 1 to 5. Power creep is added so the spread becomes 2-5 because 1 is so easy that people don't bother with it. Repeat, repeat, now the game is so easy that new systems and/or difficulty ranges have to be tacked on to the top as happened when CoH's difficulty system was changed, then with incarnate enemies and now with Aeon TF Relentless mode.
How many people do you know or play with that leave the diff on +0/x1 for a character's entire leveling life?
Further power creep just adds to the problem forcing the need for higher diff levels and content specifically due to all the calls from the playerbase that "the game's too easy."
Edit: I didn't consider the speed-running crowd when I posted that chart or with this post. So, yea, people are still running at +0 when the only goal is the TF reward but thanks to power creep, they're completing said TFs faster and faster.