The obvious reason is the classic chicken and egg one. Not so many Titan builds because there's limited aftermarket support. There's limited aftermarket support because there aren't many builds.
The bigger issues are related to where the Titan is sold. A lot of vehicles that aren't so popular in North America (Rovers, Cruisers, Pathfinders) get decent support because they're quite popular in other markets. The Titan is only sold in the Americas (last I checked) and outside of the U.S./Canada, they're a heavily taxed, large displacement import.
Also, there's the issue of the Titan's drivetrain. Although the VK56DE engine is quite robust, it's also quite a lot more complex than most other truck engines. I recall reading, back when Nissan and Chrysler were in talks to relabel the Ran as a Titan, that the VK had as many as three times the components as the Hemi.
All of that said, if you've got the desire and the means to do a Titan build, it is a solid platform. I like it much better than the Tundra in many ways, especially both its interior and exterior styling.