The programs are designed for different purposes, function differently internally, and thus have different costs.
For instance, SENTRI was designed specifically for use at the Mexican land border, and most people who have a SENTRI card are U.S. or Mexican citizens who cross this border frequently.
On the other hand, Global Entry is only really useful at airports, where it expedites entry by allowing people to scan their passport at a kiosk rather than talk to an officer for immigration check (though of course everyone still goes through customs). There are no such kiosks at land borders.
Also of note, for many people Global Entry has largely been superseded by other programs, such as Automated Passport Control (which lets US and Canadian citizens and repeat VWP travelers also use kiosks for immigration control) and Mobile Passport Control (which lets US and Canadian citizens do immigration control from their smartphones), both of which are free.