Why sub carrier spacing is more for wifi than lte

Question

I know 15 Khz spacing is used between between two sub carrier. From where this 15 Khz is derived ?
What things are considered to calculate this number ?

Answer 1

I thinks it is because of orthogonal frequency usages in LTE which make sure at any point of time only one sub-carrier can be at peak. Which minimizes the sub-carrier frequency interference.
Let others comment if anyone has some extra information.

Comments

In LTE , cellular signal coverage is more, so more delay spread-> More ISI-> More freq selective fading due to less coherence BW. Coherence BW is inversely proportional to delay spread.  
To make channel freq flat channel, sub-carrier BW must be less than coherence BW

In wifi , delay spread is less due to less area coverage