Part of the appeal of mister is it runs on cheap "dev" FPGA boards that have their price reduced to appeal to students and educators. Those are typically boards from Intel and Xilinx. It's been awhile since I looked at open FPGA synth tools, but I remember that Intel support was non-existent and Xilinx support was rudimentary. You couldn't get the same performance and packing as you can with the proprietary tools.