I just replied to a very simular post to this one so i´ll just copy paste my reply from that post in here.
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.
Of course this is accomplished by taking my arena defence out and putting it back in after a day or two and repeat as to avoid getting close or into platinum meaning my arena defence consists of either a 1 champ free win defence or a previous platinum arena defence depending on if i want to lower or raise my rating ( dont want to be pushed into G3 by accident )