From the perspective of the lore: portal storms ended after the 7 Hour War, Nihilanth was killed by Freeman, and Xen is under Combine control.
From the perspective of the gameplay: it was primitive and might annoy players because enemies could spawn directly in front of the player and instantly kill him, forcing the developers to waste time fixing that issue.
Valve, during the development of Half-Life 2, decided to forget about Black Mesa entirely. This is why, in Opposing Force, the G-Man activated the nuclear bomb, destroying the entire facility
From the perspective of the lore: portal storms ended after the 7 Hour War, Nihilanth was killed by Freeman, and Xen is under Combine control. From the perspective of the gameplay: it was primitive and might annoy players because enemies could spawn directly in front of the player and instantly kill him, forcing the developers to waste time fixing that issue. Valve, during the development of Half-Life 2, decided to forget about Black Mesa entirely. This is why, in Opposing Force, the G-Man activated the nuclear bomb, destroying the entire facility
but portal storms started again in ep2 and not like every npc needs to be teleported, just a few for special moments
These portal storms of the large portal are too located, and it might just be a reaction to something when the big tornado was opening a big portal
Or this portal was closed, which is why it was just destroying everything with its portal storms