system
Given the following post, respond as if you're commenting on Reddit:
human
[Half Life/Portal] Why would Aperture design a portal gun when Black Mesa already has far more advanced portal technology?

The portals in Half Life weren't limited to flat surfaces within a small enclosed place. There was even a portal gun that could teleport users to Xen.
gpt
Short answer? The Black Mesa portal tech only connects to Xen. 

The Portal Gun creates a stable and safe(ish) wormhole between two points, without having to bounce off any exotic universes. Kleiner's later research did succeed in producing (semi)reliable linear teleportation, but this actually seems to be a very uncommon thing in the HL universe. 

In fact, this technology has eluded the Combine, who rely on Black Mesa-style gating to "bounce" off Xen.

If one subscribes to the various G-Man conspiracies that suggest he was instrumental in the development of Black Mesa's portal research, it's even possible that Black Mesa's technology was more or less entirely based on already available Combine technology.

So:

Black Mesa / Combine Teleportation: A -> X -> B

Kleiner Teleportation: A -> B*

Aperture Portal: A <-> B

EDIT:

\* Kleiner's teleporter technically still uses Xen, but it does not require a stop there.