The storms were designed to bring new variety to null sec space and the gameplay there. And sometimes low and high sec systems if they’re connected.
Pochven is disconnected from all other types of space. If the metaliminal storms were in Pochven instead of null sec, null sec wouldn’t notice a thing and Pochven would be both a new region and a place where storms like those are.
There might be a weak lore reason (trig storm, trig space) to have the storms in Pochven instead of null sec, but CCP made a gameplay reason to release those storms in null sec.
Wormholes are random space by design, as every day your connections are to another part of space. On top of that, Wormholes have some holes which have system-wide bonuses and maluses to bring variety. Pochven, due to the nature of the entrances into that part of space (wormholes, filaments) also has a big random factor to living in that part of space.
Low sec, high sec and null sec are largely static parts of space. CCP wanted to shake things up and add more randomisation to those static parts of space where every system is the same.
High sec and low sec now has more variety after trig invaded and edencom invaded systems have been introduced. While it is static, each of those systems have their own types of enemies and system-wide bonuses and maluses.
Null sec was largely untouched by the invasion. So CCP added randomised storms with random bonuses and maluses to null sec, spilling over into low and high.
Why not Pochven you ask? Because CCP wanted to bring a random element to gameplay in null.