The reason is very similar to why people from United Kingdom get upset when you call it England. Czechia is at same time name of the main region and (now) the country as a whole.

Funnily enough the distinction between regions exists in english language - Bohemia being the name of the region. So Czechia as name for country could work. But in czech language word for Bohemia region is "Czechia" (there is no Bohemia).

So thats why for years you had people insisting on the Czech Republic. Because you don't want to overlook the other two regions Moravia and Silesia.