First, I don't believe that "why" is something that this community can answer; this was a decision of the designers and their reasons, to the extent that they have any, are a mystery.
Notwithstanding, your question is why the XP per level looks like this:
![XP per level][1]
Steady growth until 11th level, then a sharp drop and not reaching the 10->11 level again until 14->15.
However, the XP values are only one side of the equation; the other is how much XP is gained per encounter (p.82 DMG). Ignoring modifiers, by combining these you get this chart:
![Encounters per Level][2]
Easy, Hard and Deadly encounters are approximately 2/3, 1.5 and 3 times a Medium encounter (presumably because of rounding off). Focusing just on the "Hard" encounters it can be seen that you need 4 to reach levels 2 and 3, 8 to reach 4, approximately 10 for levels 5 through 10, 11 for level 11 and then about 6 to 7 for levels 12 to 20.
In this context, the jump at level 11 is only about 10% and then it falls to a much lower and approximately constant value.
If I were to speculate, and I will, I would guess that the design intent is to:
- Provide relatively rapid advancement through the early levels.
- Slow down this advancement in the mid-levels (4-11) to an approximately constant level of about 10 medium encounters (noting that difficulty factors will make the actual number of encounters 2-3 times this).
- Provide more rapid advancement (about 1/3 quicker) for the levels 12-20.
This accelerates the PCs through the fragile early stages and provides rapid gratification, provides a long period of play in the mid-levels, suitable for the dungeon-grind and then move more quickly through the levels where nation and world shaking events may be happening. [1]: https://i.sstatic.net/ezsDF.png [2]: https://i.sstatic.net/BoTgI.png
