Main reason is that DAPLINK cannot be used to debug non-arm core according to the license of CMSIS-DAP. And the JTAG support isn’t included in the latest official cmsis-dap implementation.
If someone has a standalone debugger running daplink firmare with JTAG support, he can use it to debug RISC-V core just for try and test by changing the interface from jlink to cmsis-dap in the .cfg file under boards/rv32m1_vega folder.
Regards,
Howard