Gran Turismo was Sony's first IP to sell more than 10m copies with an individual installment. What this series brought to the table was a previously unheard amount of official car manufacturer licenses and for a long time realistic racing games were among the most popular measurements for the visual ceiling a given console was capable of.
The popularity of the subgenre as a whole declined during the 2010s, not because of lacking quality, but more likely because simulations let people know what to expect and that is that it's not particularly exciting to keep driving laps with only rare takeovers occuring. The improvements became so incremental eventually that new games began to feel redundant and sales suffered as a result. Another contributing factor might be that nowadays a wider range of games is able to provide a satisfying level of realism, unlike during Gran Turismo's heydays.
What's your favorite Gran Turismo game?
Bonus question: What are your hopes and expectations for Gran Turismo 7?