One of the characteristics that gets looked at anymore when considering a new kit of any model car is what I would term "versatility". That is, does the subject have some "legs" to it, or can it stand alone, as a "one hit wonder". By "legs", I mean can the subject be presented in different "roles" from say "factory stock" to drag racing, to Nascar, perhaps even a street rod, or does it have enough popularity to make it sell successfully just showroom stock.
Just the sheer number of kits that must be sold in a reasonably quick time from introduction practically dictates the type(s) or make and model of cars that make good subject matter. How many? Well of course, model kit manufacturers are loathe to reveal numbers of that sort, but rest assured, it's many 10's of thousands of any one model car subject. It takes sales of such a magnitude to replenish the bank account after paying for all the development and tooling that goes into any new model car kit--and that kind of money will still buy a very nice house, or a VERY serious exotic sports car! Now, 50-some years ago, model companies almost could not miss--the market for model car kits was made up largely of kids, mostly in the age range of say, 10 to perhaps 16 or so (you know,that age when guys discover real cars and girls), who gladly scraped together allowances, paper-route and lawn mowing money to fund trip after trip to the hobby shop, variety store or wherever they spotted model car kits. Considering that in those years, several 10's of millions of potential model car customers having very little in the way of "competing" activities (cable TV had yet to be invented, computers were massive consoles with flashing lights and bearded scientists in white lab coats, electronic and digital games had yet to be dreamed of--all of that). Model companies were not that far out of their infancy, so the choices of kits was a lot smaller than it is today and model kit manufacturers were virtually guarranteed that whatever they tooled up to squeeze molten styrene plastic in would sell, and often in HUGE numbers.
Contrast all of that today: While kids certainly do build model cars nowadays, their segment of the market is much, much smaller than it was in say, 1963; the bulk of the model car kit market being adults--the oldest of whom still remember being teenagers in the 1960's and 70's. In addition, where a half-century ago, in any given month of the year, there might have been 50-60 model car kit subjects available, today there are several hundred, if one takes into consideration not only the American brands, but also model car kits from Europe, Japan and of course Korea and China (hey, there are even model car and truck kits being made in Russia!). All this means that any new model car kit subject that reaches store shelves has to be able to make an impact sufficient to pull sales from all those other competing products. Any time that a new model car kit can be laid out so as to be able to produce several versions of the actual car, that's a huge plus--often that alone is the "make or break" or "go-no go" decision point. A 1956 Thunderbird is a 1956 Thunderbird, about the only option being to include the lift off hardtop or not, while a '55 Chevy came in several different body styles, and I've not even mentioned the concept of a racing version. In the latter case, provision must be made, going in, to make possible future variants--once that steel has been cut, it is both expensive, and rather "iffy" as to the idea of doing even a second version "after the fact" can mean very expensive tooling alterations which may or may not be possible to undo later on (Revell's '57 Country Squire of 55 years ago comes to mind--the tooling was irrevocably changed to produce a Ranchero a year or so after the Squire was put on the market. So such planning almost has to be done from the get-go. And these are but a few of the decisions that must be made, and of course, the subject matter being considered, and how that might be produced weigh in very heavily indeed. Too many kits generating mediocre sales and the company may well not survive.
So, today much more than in the distant past, careful subject selection is key to the success of any model kit manufacturer.