I would argue that the user interface was a conscious choice by the developers. When your tools are limited, there's a much higher "skill ceiling" (the best you can get) than when simple things are handled for you. In Starcraft, you have to make conscious decisions on what to focus on. Do you want to make sure that your economy is strong and that you're constantly producing fighting units (macro)? Do you want to manage your units and get the most efficiency possible out of them (micro)? Do you want to focus on harassment, positioning, laying traps, etc? Can you do all of these things at once?
There are even Starcraft players who think that Starcraft 2 makes things TOO easy. While this is an April Fools joke, there are plenty who actually would WANT the game to be this hard. Sure, it would be nice to be able to tell a Barracks to "constantly make marines, but don't take money until you need to start a new one," but then Macro becomes obsolete, and there is no skill to being able to make units. Some people are great at Macro, some are great at Micro, and some are good at plenty of other things, but it's this diversity in HOW individuals can be skilled that makes Starcraft interesting. Some people actually ENJOY it.
I think that Starcraft, while being a strategy game, has a lot in common with twitch-based games as well... enough so that it's "more" real-time than other RTS games. Some people prefer slower-paced more strategic games, and that's fine, but they're not "better" any more than Chess is "better" than Starcraft.
As to why people like to watch it... well, I think it can be a combination of things. I watch the GSL, and I've even gotten my girlfriend into it. While I enjoy it because I know the sort of skill that the game requires, my girlfriend really enjoys the casters (Tasteless and Artosis), who are quite funny, and the players (along with the rivalry and ceremonies after the game). She doesn't really understand the strategy, and probably can barely identify a "6-pool" (an extremely early opening that aims to do tons of early damage), but she still enjoys watching it because the game is just straight up entertaining to watch. I also expect that a lot of the appeal is in HOW popular it is now, and how it can be fun to watch something with a group of people and cheer when other people to cheer (my response to football).