It's very hard to figure out the problem without knowing some more information, but here are a couple of tips to keep in mind:
Not enough tourists to your event center is usually due to traffic. (Almost everything that annoys you about this game is ultimately about traffic). Take a good long look at your traffic flow; are the tourist Sims coming in to the city but not making it to the event on time? Are they all driving in, then clogging up the roads afterwards? There is a good discussion with a lot of tips and tricks on here, you may want to read through to see if you can gain some insight. Here's the most recent: https://answers.ea.com/t5/SimCity-2013/Traffic-Solutions-please/m-p/7160232#M127154
Whichever event venue you are using, make sure you have regional transport set up near by. You want the correct type of transport for the wealth class of the tourists; remember that High Wealth Sims don't ride buses, so if you're running a HW event, a bus terminal won't help. Use ferries, streetcars, etc.
Set some Commercial zoning around your event center. Tourist buildings and landmarks will cause the commercial buildings to turn into hotels as you ramp up their density. Having hotels in the area will mean that your tourists can walk to and from the event, and won't clog up the streets with taxis when they leave.
Plopping a landmark with a small footprint, like the Eiffel Tower, close to your event center can help draw the tourists in before the event, and then they can walk to the event once it starts. This also helps to get those hotels growing in the commercial zones to cut down on the traffic.
Hopefully you'll be able to tweak a few things and figure out what's happening. Happy Building!
