The most common reason for this is their Filter settings.
The most often overlooked Filters are these first three. Click on them to turn them On/Off. When they are On (green) they remove servers from your list, so turn them Off in order to see the most servers.
1) UnLocked - when this filter is On (green) it removes any servers that are locked / password protected
2) Server - when this filter is On (green) it removes any games not on dedicated servers (ie. client-hosted games)
3) Ring - when when this filter is On (green) it removes any servers that are not running "Classic" gametypes
The rest of the Filters are accessed by clicking on 4) the Filters button.
The remaining filter settings are self-explanatory and the best settings for seeing the most servers are:
10) YES (this filter is only available in Custom Edition)
The only other way that you might see more servers is to install a mod like HAC 2 which lets you see servers running on other Halo versions (eg. 1.00 thru 1.09). That should be less of a problem with version 1.10 because most servers will be listed in the 1.10 Lobby after the GameSpy master server is shut down.
If you don't see ANY servers in the Lobby or if you get an error when you try to open the Lobby then you are probably still using a version of Halo older than 1.10. After GameSpy shuts down on May 31, 2014 you will need to update to Halo version 1.10 to use the new Lobby service that replaces the GameSpy one.