PD told us when they launched GTS, that the car list could eventually reach 500 cars. That didn't happen, and we won't even reach that number with GT7 at launch (from what they said so far). Once they stopped delivering new content, the number of players playing the game regurarly, decreased. Right now, unless they started adding the content that we're getting with GT7, 2 or 3 years ago, it wouldn't be as profitable as selling an entire new game. Plus, being a numbered GT, has more chances to sell more than GTS, which some people didn't buy because they thought it was some sort of Prologue.
In the end, it's always a lose/lose for PD. Releasing the game exclusively on PS5, would generate complaints due to the inaccessibility of the console, releasing it as cross-gen generates complaints over the quality of the game, updating GTS with paid DLC would generate complaints about PD milking every last cent from GTS's playerbase, when they should develop a new game. Of all the 3 lose conditions, they chose the one that grants more profit, which, in a business perspective, makes sense.
In the end, of course I would like to have a constant flow of new content in GTS. But since they started diverting their resources for the new game, I much rather have a new game in March, which will cost me around 70€, than buying DLCs for an older game, that will eventually add up to a similar amount of money, plus having to buy a new game anyway 1 year later.