While a 10/20/25 minute limit is a way to cheat import taxes on video cameras (devices that can record at least 30 minutes of video), there's a chance there is also a technical reason...
When the 10 minute limit was removed from the D3100 (the then-cheapest Nikon reflex, with all other video models going for 20 minutes) , we found out that it crashed after about 17 minutes because a variable holding frame count was 16 bit and started overflowing!