It's interesting, isn't it? Heuer's focus in the 70s was clearly on the Cal 11/12, and manual watches provided a cheaper alternative so the Cal 15, as a cheaper automatic option, was already in something of a niche from birth. For a Carrera, it (i.e. the 1553) was priced at 91% of the price of the Cal 12 1153, whereas you could have a 3-register 73653 for just 83% of that price, a 2-reg with date 73453 for 78% and the plain 2-reg 73353 for... well, for less. The Vj 7733 models don't often appear in price lists - either they weren't offered in some markets, or you were being gently encouraged to buy one of the dearer watches :)

Judging by watches I've seen over the years, all that doesn't seem to have done the Cal 15 a great deal of harm in the Carrera and Monaco - I've seen plenty of 1553s and 1533s around. I haven't seen as many Cal 15 Autavias though and most of those I have seen were 1563s rather than 15630s. So on that anecdotal basis, it may just be that there aren't that many of them around.

Attractive watch though, this photo borrowed from a post by Peter Moller: