Ssssooooo many answers for one simple question. I could type on forever but I won't, that would take too long. Simple reasons are....
1. KOF is the series that helped to re-invigorate many fighting game fans. I am included in this bunch. I had grown somewhat tired of fighting games until KOF 94 came out. Team Battle changed the face of fighting games forever. For better or for worse
2. So many characters to master over the years. I have been playing KOF since 94 and I can not honestly say that I am proficient in combat with even 70% of them
I learn new things in each respective KOF all of the time. Wether it be pertaining to combos, story, move priorities, or what have you wink
3. KOF has always had an unusually strong storyline for a fighting series. The Orochi saga, the N.E.S.T.S. saga, and the relationships between characters are unmatched and untouchable in a fighting game series.
4. A large number of characters represent a large number of fighting styles! If you like grapplers you can find a character you like, if you like hit and run type characters you can find one you like, if you like basic easy to learn OR hard to master characters there is someone for you. New styles are represented almost every year. Just look at Angel. 8 years into the series and KOF gives us something that we have never seen before
(shameless Angel plug BTW wink )
5. Gameplay changes quite often, no matter what some people say
SNK, and now those who carry on the legacy, have managed to change the type of gameplay from year to year without becoming stale! Team battle in 94 as stated before was revolutionary! SNK has given us strikers, BC attack mode, dodging, wire attacks, HSDM's, and many other gameplay innovations
6. The amazing satisfaction of beating a KOF boss for the first time. All of them are great but some of them leave you feeling exhausted after a long fought battle and a well deserved victory! *looks at Goenitz* KOF bosses can not be touched!!
7. IT'S KOF! LOVE IT, LIVE IT, RESPECT IT!
Q-
Heh, guess I did go on for awhile huh?