Basically, the issue NI would find itself in if they supported FPGA dev board X (which NI doesn't sell), is that another user would ask for support for FPGA board Y and Z (also, not sold by NI). Since the support for these boards includes some use of peripherals hooked to the device (buttons, LEDs, other IO), it quickly would become a burden to support.
This issue becomes that having reasonable LabVIEW FPGA presence (e.g. each of the peripherals available as FPGA IO) requires non-trivial work to create for each board, plus there's need to create an installer, do some level of testing, etc. etc. etc. Multiply this by the number of boards supported.
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