There were very few people boxing before EQ1 hit Planes of Power. Funnily enough, while on forums like this boxers were anathema back then. the real growth in people boxing started with the high end guilds. Then the endless nights wiping trying to beat various of the Planes produced burnout in the Clerics before anyone else. From then on high end guilds were forced to use boxed / shared / alt Clerics more and more.
The introduction of AA"s, people realised that with a couple of boxed chars you could get AA"s far faster than in a party - plus you didn"t have to hang around for an hour or two LFG (unless you were an Enchanter, who had always had the option of soloing using charmed mobs). The number has grown exponentially since then.
I 6 boxed in EQ1 and 5 box in WoW and EQ2. I also spent / spend time with raids and partying. I was in one of the top guilds for a couple of years, working with the EQ1 devs (as a guild, not personally) on end game content, so its not because I"m anti social.
To answer your question, I got into it originally because I got fed up LFG in EQ1 for hours on end. Now I enjoy it because you can enjoy instances much more doing everything at your own pace and I enjoy exploring, plus its a challenge. If its possible I want to do it and beat it. I still like soloing, partying and raiding too. I"ve never "farmed" and I"ve never sold anything for RL cash. I just enjoy boxing.
i don"t know about "very few" before PoP. people in my guild and friends of mine were boxing since kunark. in fact, i knew a decent amount of people on my server who boxed almost since the beginning. i agree with you that the introduction of AA"s lead to more boxing because people realized it was faster to get AA"s while boxing, but i think it was something that a lot of people were doing long before planes of power was released.