Alfa occasionally does unrecorded things, so I hesitate to use the word "always" where they're considered. However, the dogma is...
Alfa could not meet emissions in the US with carbs, so they ended up installing Spica on the 1750 and 2000 cars intended for the US market. The 1300 and 1600 were not officially imported during those years, so no need to install the injection.
When I was an Alfa parts manager in the early 70s a new Spica pump had a list price of slightly more than a same-year new engine. So, there was no motive to use Spica low-margin cars.
Yes, it was fitted also to the Montreal, which was never offcially imported, and (based upon gas consumption if nothing else) is assumed to be unable to smog-certify in the relevant years. It was also used on the somewhat related 33 Stradale and T33 Race cars, so we might assume that Spica was useful for maximizing horsepower when that was the mission.