Pretty technical question. So, we'll have to have a pretty technical answer.
Bose-Einstein condensation occurs when matter is cooled down, and there are just not enough many 'states' for all the particles 'to fit'. So, being bosons, which can squeeze into the same state, any excess bosons end up squeezing into the same, ground state. This is what is meant by 'Bose-Einstein condensation'.
So why doesn't it happen with photons? Because there is no such law as 'conservation of photons', they can be created and destroyed at will by interacting with matter. So, when you try to cool down a photon gas to achieve a B-E condensate, they just get absorbed by the material they are in contact with. Therefore photons can not go under B-E condensation.
They do, it's called a laser, where the photons all have the same wave-function, except it's not REAL Bose-Einstein condensation. The difference is that no phase transition takes place in a laser. A gas of rubidium-87 atoms say, can be made to condense into a B-E condensate because as they are cooled below the critical temperature, the chemical potential goes through zero. Lasers on the other hand don't 'condense'.
Hope that helps You, Good Luck
