imho, it had potential to be significantly less horrible (I could possibly even go to the extent, dare I say it, maybe even 'good') had someone with a fraction of a brain written it. She is just too noob really, the level of writing and the 0 character development lead to the poor reception amongst intellectuals. However it was successful and (somehow) managed to acquire a large amount of support and fans.
As for your question why someone would think twilight is 'good', I cannot help you there. The best thing I can say about it is that I found a portion of the plot 'ok', but it doesn't even begin to outweigh lack intellectual stimulation and general shittyness of how it was written.