Because it’s not a traditional forest, it’s a desert. We had a wet winter here, which allowed non-native invasive plants/grasses to grow abundantly. Those grasses are dead, dry fuel now, allowing the fire to burn hotter than it would normally, which makes it harder for individual specimens to survive the burn. The intense fire heat also changes the nature of the soil itself, causing more water to run off rather than being absorbed. Desert topsoil in this region has a unique, delicate balance already, even footsteps or tyre tracks can disturb the biome for decades.
Compounding that, climate change has already shrunk the area where slow growing Joshua Trees can even survive. The trees lost in this fire will not grow back. You can still easily see burn scars from the 90’s in Joshua Tree National Park, which is adjacent to the Mojave National Preserve.