The reason arena is harder is partially due to champion fusions such as tormin and rotos who caused major imbalance in arena. The reason it causes imbalance and affects even players at lower ratings is because players such as myself who previously played in platinum prior to the release of Rotos stopped giving a fk about platinum all together due to how extremely specific the mandatory champions are.

What happens at that point is that im taking my platinum geared champions into gold 4 where i can win without effort and simply farm medals which leads to the people who were previosuly in gold 4 to be pushed down to say gold 3 where the cycle keeps pushing downwards.

More players reaching endgame and endgame gear and such also causes this effect but the main problem is game balancing or more lack there of which causes alot of players who would otherwise be competitive to stop trying simply because they dont have that one or two champions that are now mandatory.

I don't think it is specific fusions, just that the "population" skew is older. There are just more people that have been playing longer, better champs, better gear. The only solution is to adjust the size of the tiers to reflect the current distribution. That is make the gold tiers larger. For example,  if the size of gold 4 was doubled so everyone currently in 3&4 were in 4.  That may be going to far, but hopefully that makes sense to someone.