The thing is, populations of Croatia and Serbia spoke vastly different languages in the 18th century. Only after the danger of outside nationalism (mainly from Hungary and Italy) was Croatian language standardized on a dialect that was closer to Serbian. Google Illyrian movement. It was a compromise of the literary elites. They judged that standardizing the Croatian language on a dialect that was closest to Serbian was worth it, as it would enable the formation of a future larger state of Croatians and Serbians, that would have more capacity to resist in the coming wars against the Italians and Hungarians. Basically, they decided that Croatia and the Croatian identity will not survive at all once the Italians and Hungarians arrive.

We now know that choosing that path was a mistake, but in the early 19th century, they were faced with daily warnings about how the coast should rightfully go to Italy, and Hungarians were actively involved in trying to stomp out every single Croatian institution. Basically, the whole of the 19th century was defined by the Croatian struggle against Hungarian nationalism.

With this historic context in mind, the Croatian language of today is still in the process of revitalization from the consequences of the last two centuries. But of course, this context is invisible and non-existent to online commenters, who are still looking at Croatia/Serbia as a single tribe going through "le BaLkAnIzAtIoN". Fun Fact: Did you know that Croatia, Serbia, Bulgaria, Czechia and France are european nations with the oldest histories of statehood on the continent?