To put it simply - I think the low turnout was because people were afraid of a terrorist attack. There had been enough media rumours of the same to scare the public and very little to show in terms of prevention strategies - in fact the dominant perception was that "Team Anna is wholly responsible for the event". And though there was hollow talk of security being provided...all TV and newspaper clippings only showed tired policemen with BATONS (err - which era are we in?) standing around. There was NO X-ray machine, metal detector or entry-level manpost (for frisking) in sight.
(Correct me if I'm wrong, I didn't attend so I'm only basing my views on media reports.)
Had I been interested in going (I'm more of an Internet and coffeeshop activist...ahem...plain lazy) I would have feared for my life knowing Mumbai's history with terrorist attacks.
And I believe the rest of Mumbai's interested public thought along similar lines...