Well tanks held all the power in TBC because there was no LFG, you couldn't just que up for a new tank. If a tank left the group was over. There was also no cross server dngs it was pretty easy to write down someones name and avoid them in the future, this limited your options of tanks but it also meant you didn't have to worry about a tank leaving. However tanks are very emotional people, you had to tip toe around their every whim or you risk upsetting them and then yeah they leave.
This happened in raiding guilds too, if your tank left you were basically screwed because in TBC most guilds geared up the tank first and if one left and you were working on Kael'thas you weren't raiding anymore. Which is why the whole "Raid leaders/GM/Officers are usually the tanks" became a thing because those 3 positions are less likely to leave a guild over a random trial/member.