answrs

[Darkstride]

The original game (A Realm Reborn) was built around level 50 as the cap, so that's the extent of its content. Heavensward increased the cap to 60, adding a lot of new, higher level content and continues to release more.

[ChameleonMS]

You character level is just the beginning. Your item level, is the true progression.
During 2.0-2.55, maximum character level was always lv50. item level progressed each patch from i70 to i130.
After Heavensward, 3.0, the character level cap was increased to 60 and iLvl cap was increased to i210. Since then, with 3.4 around the corner, i240 is the current cap (going up after next patch).

The items levels are game restriction to harder content.
Using an example from back when you last played Titan Extreme requires a character level of 50 and iLvl of 67; just to try it.
There 8 player trials, 8 and 24 raids, 4 player dungeons, and a ton of other stuff.