Easy to have the best selling car when you sell very few models across your range. Other car companies "dilute" sales of individual models because they have multiple slightly different models targeting different price points in the same segment, for example globally Toyota sells the Highlander/4Runner/Crown Signia/Prado/Landcruiser SUVs, Corolla Cross/RAV4/bZ crossovers, Corolla/Prius/Camry/Crown/Mirai sedans/hatchbacks. Each of these cannibalizes each others sales, but in toto sell more than any single model Tesla sells for a single segment globally.