I ask them to. At the end of every hub, I put a 'call to action', a question to the reader asking them what they think or a specific question and asking them to comment; for example:
'Do you have any techniques of your own for marketing, or have you had success with what we've discussed here? If so, please let us know in the comments.'
I put it in bold and italics so it really sticks out, it seems to work pretty well. (Around 90 comments over the last 3-4 weeks, around a 3% comment rate).