Tickets were pretty expensive, the visa giving situation was a total disaster, marketing was horrible. They could have pulled a lot more fans easily.
Also, I don't think we would get more fans at all unfortunately as the tournament progress (apart from the knockout and India games, which should be full houses). Usually interest tends to be highest at the start and in the end (knockout games).
With too much T20 cricket going on there is little interest in ODI. It took almost a day to watch an ODI and there are not much people who have that much time. If in India people are not coming to watch ODI world cup match then situation will be much worse in rest of the world. Apart from India games there won't be much attendance in group stages game.