They have some cool diagrams showing possible payloads in different fairings:

https://www.rocketlabusa.com/launch/haste/

I’m guessing the main reasons are cost and response time.

The suborbital ones are for hypersonic testing. Like, testing missile parts.

I wonder what the pricing is and whether it’s competitive with the newer bus manufacturers.

It’s interesting that LM is buying Terran while making this bus that overlaps with Terran’s bigger offerings (“Embassador” and “Enterprise” buses, apparently).