No reason they could not have done it in the first place.
Actually yes there is a reason, though it may not be one that you, I, or others like.
Revell knows that by offering certain specific parts in certain specific versions, that they can actually increase overall sales by creating a market for buying the multiple versions. This way if you want to built three different factory stock versions, (well, almost three, more on this in a moment), you're required to purchase three separate. kits, (technically more, but I'm keeping this fairly basiI).
You see, Revell, (& Round2, & all the model companies), are only interested in how many units they can move to the various distributors, retail outlets, etc. They don't care if you purchase the kit at full retail price, with a Hobby Lobby 40% off coupon, at a contest/swap meet, or however else you purchase it, once they have sold it to the "middleman". They also know that the more variations they can put out, the greater the shelf space they can command & the more kits they can move to the various outlets. A side benefit of this process is that they create a market for people to buy multiple versions in order for them to build the model the way they want to. Since NASCAR outlawed the superchargers & FI units partway through the 1957 season, a builder can use the Paxton blower from the NASCAR version on the street car, & swap the single 4 BBL from it to the NASCAR version & still build accurate versions of both cars.
My only minor quibble with them over this kit is the fact that the street version is lacking the proper single air cleaner for a correct factory dual 4 BBL "E Code" variant, meaning I had to rob one of my old AMT 56 Ford kits for the proper air cleaner, but I didn't waste my time complaining about that; I simply took the correct part from another kit to build an accurate version. However, including that one piece in the kit would have required less tooling money than all the Paxton parts did.
Expecting them to include the Paxton blower in a kit of a car that wouldn't have had it installed anyway, (as in the police version), is a bit ludicrous as it is. BTW, that's also why this is a limited edition. To Revell, it wasn't worth the extra cost to tool up the needed parts to do a police car version on their own, but with Dave Burket paying for the limited run, it was more cost effective to do it that way. The number of police car versions of the kit Revell would have had to move to show a profit likely means it wasn't worth it to them to do that. The Model King can pay them upfront for the limited run, as well as footing the bill for the extra tooled parts, as he has done in the past with other limited runs from the manufacturers, & till turn a profit, & Revell has the additional tooling paid for & in their possession if later on they decide to do a police version themselves.