Why is it so difficult Ranking up from Bronze to Crystal

[Loggos]

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I agree with this. And I think another point is that PvP's reward psychology is different from ultimate reward psychology.

Conceptually ultimate is all-or-nothing. You either clear it or you don't. Comparing progress to ranks below crystal is a contrived comparison in my opinion because ultimate has one goal (clear it) and mere progress means you haven't reached it. It's cool that you got this far but you didn't succeed at the one thing you needed to do.
It's also a lot more about long-term team success, clearing one thing together after weeks/months of work.

PvP ranking is different. Unlike ultimates, personal progress in PvP is the end goal.
It is a perfectly reasonable motivation to reach a rank below crystal. You didn't fail your one objective if you haven't reached the latter (and some people might never reach crystal anyway).
It's a lot more individualistic because PvP is about measuring nothing but your own growth/state vs. the long-term team aspect of ultimates.

To make a cheesy comparison, ultimate is a lot more about the destination (clear an ultimate and then the next when it releases) and PvP is a lot more about the journey (steadily become better/reach a higher score across a continuous time frame).

That personal journey is also reflected in the fact that it can take several seasons for somebody to become good enough to reach crystal but you'll still have a feeling of success while you progress through the ranks over time.
It might also take you several ultimates until you manage to beat (a current) one, but the progress experience is different. It's not "I reached this new official milestone and maybe next season I can score even higher", it's a lot more binary: "no success, no success, no success" until you finally have a "success".

I don't know what the best reward system is so I don't want to comment on commendation crystals in particular but I do agree that PvP's reward structure should better reward the journey/the personal progress which lies at the heart of its mode.

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(Yes, ideally playing pvp/progressing should be a reward in and of itself and a lot of pvp regulars are intrinsically motivated. But imo in its current iteration CC is just not appealing to people who might potentially enjoy ranked PvP but aren't sold on it yet/have little experience. If you are just starting out or stuck at lower ranks your experience might be so miserable you associate nothing but negativity with this mode.
So I think especially at the start/at lower ranks small but steady experiences of success are important to keep people engaged until they are in a place where they can extract feelings of success from their progress itself and the satisfaction of getting better has turned into an intrinsic motivator.)