There are any number of reasons behind it, and without someone coming out and saying exactly what the reasoning is everything is just speculation, however my theory would be, looking at the current event for example.
The prizes build up from small ISO and recruit tokens which anyone can get without really trying for the first 200 points.
Then they improve from 275-600, a daken, some heroic tokens and more ISO and some hero points.
600 in these events is a fair amount and takes a good effort to do, but most people should be able to work at it and reach those marks if they try.
It is a good incentive for all players and it lets the lower level players get their hands on some decent stuff to help them improve without giving them away for free and without giving them really awesome prizes.
Then it plateaus. Personally I would say this is deliberate, they have given away some good stuff, the gap from 700- 1100ish is filler, some iso and stuff, its a reward to say “grats you have reached another milestone, keep going the really good stuff is getting closer”.
Then the REALLY good rewards are from 1200+. They are hard to get, they need real effort, lots of hero points, and 3star rewards. Only a tiny number of people will ever get to this sort of point, and it will require time, resources and tenacity. The rewards are good because you have to earn them.
Ultimately its all about psychology and understanding and manipulating people to play, spend and try and reach the goals that they dangle at the end ever so tantalising.
I think there is a great deal of careful thought that goes into the prize structure, not saying its all right or they do it correctly every time, but that my, somewhat cynical, analysis.