How did fetishisation of Rhodesia become so popular among far-right groups in the West in a way that apartheid South Africa never did?

Why groups romanticize certain figures and events over others is a topic itself worthy of in-depth research. I for one would suggest that an answer to your question is somewhat more grounded in psychology than observable history for several reasons. Not least of which is that it more often than not involves an a-historical understanding of events. Since the late 19th and early 20th centuries, you can see this with the "Lost Cause" myth of the Confederate States of America and the American Civil War perpetuated by groups such as the United Daughters of the Confederacy with a political interest at stake. So too can you see this in regards to Rhodesia.

The romanticized view of Rhodesia you're talking about has its roots in deliberate actions discussed in far-right internet forums such as Stormfront. Rhodesia is a useful narrative for them to use because most of their target audience (disaffected white male teenagers and young adults) have otherwise never heard of Rhodesia. At least not enough to be able to form an opinion on it. The Confederacy, Apartheid South Africa and most glaringly the Third Reich are well known enough that the far-right narrative is unlikely to shape a 19-22 year old's understanding of it. Whereas Rhodesia is a blank canvas for them. You can see this same tactic being employed with Pinochet. Hitler, Mussolini and to a lesser degree Franco are all too well known to be convincingly portrayed as just to someone new to the far-right. At least in the beginning of their indoctrination. Whereas most kids in North America and Western Europe don't know anything about Pinochet.

Returning to Rhodesia, the far-right "meme-centric" understanding of its history is reliant upon a mythologized, biased oversimplification of history. The broad strokes of "Rhodesia was a prosperous British colony that sought its own independence and was thus ostracized by the global community and besieged by communist guerrillas" is not inherently wrong; but rather is an overtly biased oversimplification. For someone that doesn't have any prior knowledge of the events, this narrative is more or less confirmed for them should they chose to do a cursory skim of the Wikipedia page.

Making the alt-right view of Rhodesia all the more seductive is the problems that plagued Zimbabwe after the end of Rhodesia. Problems which in the narrative of alt-right propaganda, validate their beliefs. The despotic rule of Mugabe, the economic mismanagement and the efforts taken to address the overwhelmingly White control of the nation's land, resources and wealth help romanticize the "doomed struggle of the noble Rhodesian who warned this would happen." Of course, history is much more complicated and the hardships of Zimbabwe can attributed to a number of factors. Not least of which are the very consolidation of economic power in the hands of a white minority and the war fought to preserve it that far-right groups fetishized.

The most frustrating element of all this is that it appears to have worked. Not only among the far-right, but also among more moderate conservatives who have likewise fallen into the Rhodesia meme trap. Predominant among whom is the case of American gun culture as detailed by the New York Times.