[Why Dose Nat Type Affect Warframe So Much]

Why dose NAT type affect warframe so much, i dont get why its such a problem for warframe when a. i can play with some one i cant join through some ones & b. why i can join others i cant join in warframe in other games.

 

for the record here is my situation, warframe says upnp is malfunctioning, attempted router update did not fix, router dose not support manual port forwarding (as of posting this yet to try to get isp to do it but have not gotten there yet) when i login warframe says i have strict nat ie type 3, how ever my ps4 settings say i have type 2.

 

i dont understand why it hits warframe so hard and im just looking for some clarification.