Same happens to me. Even donors have hacked before (either stolen account or actual owner himself). Let's look back:
-xTurtle's and other youtubers' videos showed hacking on this server before (hacking in money wars series) -> promotes cheating on this server.
-August's lenient punishment system (all of the 1 year banned cheaters got unbanned and returned)
-MineCraft has at least 25 million sales on PC (100 million total). Naturally, more players = more likely to find cheaters.
-Player base. Majority are kids and some of those will want to push the limits (e.g. hacking, swearing, etc.). Same goes in real life.
-Hackers can ALWAYS buy alts from Mojang or from an alt dispenser. From whatever source, given that there's "infinitely" many possible names (going to assume 2^31 - 1 maximum names), they'll keep coming back.
-Majority, if not all, of the hacks are PvP-related. For example, you won't find much use of kill aura in tower defense since towers do a much better job against mobs but you will find that same hack more useful in actual PvP.
-Weak anticheat. Little to no cheat protection will naturally draw a lot of rule-breaking.
My main theory lies in point #2, #4, and #6 but others should be taken into consideration too. Many other factors not listed could have contributed to the large number of hackers. The good news is that a very few cheaters (and I mean 1-5) rarely play the non-PvP games (e.g. some arcade, tower defense, and minerware to a lesser extent).