I suspect the reason is that HamAlert does not process spots for WSPR, FST4W or other beacon-style modes at this time. See also:
The performance/throughput issue in the backend has been resolved in the meantime, but since HamAlert has always been geared towards spots for actual “QSO” modes rather than beacons, starting to process beacon spots out of the blue could result in thousands of users being inundated with alerts that they don’t care about. Therefore, an additional condition type such as “Spot type” would need to be added to differentiate between CQ and beacon spots, and beacon spots would only match a trigger if a specific Spot type = Beacon condition was present.