Why does the SnAr reaction proceed by the Meisenheimer complex?
Basic level explanation: 
SN2 occurs only at sp3 carbons, where you can get backside attack.

Intermediate:
Addition to the double bond generates a highly stabilized anion. This is a lower energy transition state than the concerted SN2.

Advanced: 
The reaction occurs at the site of maximum frontier molecular orbital overlap. The HOMO (highest occupied molecular orbital, the hydroxide nucleophile) adds to the LUMO (lowest unoccupied molecular orbital,  &pi;* of the arene). To do SN2, one would need to interact with &sigma;*, which is not the LUMO, and has an unreachable trajectory (points towards the middle of the ring).