Because it was amazing?
Most of all, what got me was hard and rewarding content.. not spoonfed to players like it is now.
In a bulletlist, i'd say :
- Hard heroics.
- Hard and rewarding raiding content, with intro stuff (Karazhan - 10man), and varied and fun tiers (Karazhan+Gruul, SSC+TK+MAG, Hyjal+BT, Sunwell).
- Amazing storylines and attunements (Hand of A'dal)
- Fun world pvp on the Isle of Quel'Danas (or whatever its name was)
- Players had 'reputations' on realms, and were accountable for their actions. No bullpoop anonymity like nowadays.
- Unique flavour to classes before this mass uniformization
- Oh, windfury totem was still amazing!
- Best tier designs on most classes (T6)
- Bulwark of Azzinoth.
- Kael'Thas encounter. Awesome.
geez, why even argue.
For me, TBC is\was\probably will always be, the best expansion in WoW, nothing came even close to it.