There are actually several possible causes so let’s start with the most obvious. After replacing the front brakes, have you ensured that the brake fluid level was properly topped off after bleeding the lines? Air in the system will cause the lever to fail to return. Additionally a leaking hydraulic line or seal will also cause this to happen. These should be easy enough to spot thanks to a telltale pool of brake fluid.
If all this looks good (hydraulic disc models), check your calipers – is the piston squeezing the pads against the rotors when you apply the brake and letting them go when you release or are the brakes stuck on? This could be an indicator of caliper replacement but before that we’d remove the calipers and thoroughly clean/ grease the assembly. When it comes to the rigors of off-roading, dirt, rust and grime can find their way into the piston and prevent it from fully disengaging and operating smoothly.
If that all checks out, pop the cap on the master cylinder and check the piston on the top-side of the assembly. Pull the brake lever and pay close attention to where the lever moves the piston. Sometimes this area gets gummed up. If that’s the case, clean it and lubricate it, ensure that your fluid level is topped off to where it should be.
If all of this yields no improvement, the last thing to check is the lever itself. Look for dirt or grime jamming up the rubber grommet behind the boot – something as simple as a piece of twig or kicked up pebble here can make it impossible for the lever to retract.