In regards to why Homeworld 1 was such a critically acclaimed game back in the day (when it launched in 1999), there are a lot of reasons, but I'll just provide the main ones.
1. It's quite possibly still the only space RTS, over 15 years later, that happens in true 3D (along with the other games in the HW series). There are many other "space RTS" games out there (though most of them include the 4X title, which makes them non-RTS imo), but even most the 4X ones aren't true 3D. Take Sins of a Solar Empire for example, a popular space RTS/4X that's been around since 2008. The game was really cool and had a lot of neat elements, but it essentially happened on a 2D plane. All appearance of 3D was simply aesthetic, and sometimes it was very obviously a 2D game.
2. Being a game that happens in true 3D, without even the limitation of say hills, mountains, or other terrain features (which limits the 3D tactics of other RTS games), the player has virtually limitless options in terms of strategy, tactics, and positioning to use against his or her opponent. If you can't understand why this is so important, I suggest reading Ender's Game by Orson Scott Card as a primer.
3. The game had realistic bullet and missile physics. Most RTS games nowadays use some kind of thinly veiled pseudo-RNG system to decide of an attack lands or misses. Homeworld 1 had a physics system which would allow a projectile to pass over one target and hit another for added realism.
4. The game felt and played beautifully. It was everything you could want from a space opera strategy. However, it wasn't weighed down by the slogging pace of the typical 4X shenanigans that the vast majority of space strategy games before or since have been. Relatively speaking, whether in single or multiplayer, the games were intense, fast-paced, and free of the nuances and overcomplexity of things like diplomacy and managing an empire. I will reiterate that I haven't played another space game that has achieved this (better than the series) before or since. Most of them try to be Civilization in space, Homeworld heavily stood out from that archetype.
5. The campaign was interesting and well-done, as companies cared a lot more about the campaign portion of the game back in the '90s. It was interesting, engaging, and gave you a reason to keep playing all the way to the end.
Anyway, like I said, the series made innovations in the 20th century that haven't been replicated since. I didn't even mention several other great things like the innovative formation management or the fact that the game requires less micromanagement than any other RTS you may ever play (and yet still manages to feel engaging). There are too many good things to list about it and the whole series.
