Scarcity and legitimacy in part. The vast majority of the 1965 restrikes had low mintages of 1000 or fewer, with some as low as single digits. The restrikes were done by Artex, but the company was wholly owned and operated by the Hungarian mint, so these are considered official mint products. In general it's nice to see examples of these issues as very high grade proofs. A lot of the restrikes for more common issues can be had on the cheap (I think many are even undervalued at present) and they are quite beautiful. It's also nice to be able to view fully red examples of the early copper issues. The expensive restrikes you're referring to tend to be of mules, patterns, or exceptionally rare issues (like the 1870 gold ducat) that most collectors would never be able to afford (or in some cases even locate) an original example of. For example the 1896KB 5 Korona I linked above is a coin where I've never even seen a picture or auction listing of an original. The best catalogs I have don't even have an original specimen strike listed; only speculation that one probably exists. As the first proposed coin of the 5 korona denomination and an 1896 millennium commemorative at that, I consider it an important coin to own, but a restrike is really the only option.