Triathlons are all about personal improvement, but there are not many sports that you can sign up for a race, as just an everyday Joe, and race with the pros. I can not play basketball or baseball with professionals. There is no chance that I could be a basketball or baseball professional. There IS on the other hand a chance that someone could work hard and go from amateur status to professional status in triathlon. I do not necessarily think that I could do it, but there is a chance.
It is also not age specific. Due to the natures of basketball and baseball, once you hit a certain age you are pretty done. Triathlon has age groups to very late ages, and your potential for doing well AG wise actually goes up the older you get.
It is also not boring. You always have something that you need to work on specifically. Then you will have the next thing. Example: Last year I could not swim a full lap of the pool without stopping or floating on my back. I really worked on that, and feel as though I am an adequate swimmer now - meaning I can do 4x500 at www.luckyslakeswim.com and not drown or start having hallucinations. Now that I have that base in swimming, I am focusing on my running - which at this point takes longer than the bike leg for me. Once that is good, then I will have another area that I can focus on to make my overall time better.
Anyway, these are some reasons why triathlons are cool.