It is not written rule to progress with difficulty alongside with song time.
"Song" may be for example consistent 2 notes for 3 min, for example C->E->C->E... where by consistency rule patterns should be the same for whole map, our non written rule allows people to put illogical spike at the end "because I can".
This is also a common shit against new mappers to discourage them from ranking maps and keep old fags who can do whatever they want, for example:
morning rantMap A made by for example Monstrata, from 0 to 100% but submitted and claimed by completely unknown new mapper - forget to rank it in next few months, expect tons of mods, problems, questions etc.
Map A, literally identical same map submitted by Monstrata - it will be ranked in max 1 month without much problems.
Common reason: lack of consistency in patterns, difficulty spikes which comes to main question of this thread - why? Well.... circlejerk. There is a reason why lots of mappers who quit this or moved to less toxic mode have same opinion - STD QAT and BN should be completely trashed and renew with people who cannot be corrupted by old fags. Unfortunately peppy don't have time to deal with it and admins doesn't care at all (close seeing eyes on this problem).
Here is your explanation.
TL;DR: Because consistency is just a point to discourage people and tell them to fuck off (like I heard 2 times in a row in the past).