Why Paragon?

On why most players play as the good guy:

Boyan also noted that Mass Effect was a long series so players may play as “paragon” out of guilt, in that they don’t want the story to turn out poorly after investing 90 or more hours.

Guilt?

I don’t think it’s that difficult to understand. There are a lot of games out there. Paragon is the default. Most people are not going to play through Mass Effect or other games multiple times to get different dialog trees.