It's done that way based on ratings. I know what you mean though, sometimes it really sucks when the show doesn't do so well and the network pulls the plug while it's getting really good. I'm not sure how it's done else where but that's how it's done in the states, by ratings, that's how they get support for their shows (commercial wise).American tv drama series seem to go on forever and then when they have to end the series, you don't really get to see the two main character's story end because sometimes the studios don't want to support it anymore and pull the plug early or you've become so bored that you don't even watch the series anymore. Asian drama series have a beginning and an end, so you get a more complete story of the two main characters and the other characters around them.
I also have to add, I find Singaporean dramas interesting.Movies - American/Korean
Series - Thai/Korean/Taiwanese/Chinese