Band member distancing requirements in NL is 2m (with some additional hygiene requirements). With a big enough hall and sufficient ventilation, Big Bands can rehearse again. For concerts, the main concerns come from venues - who would need to implement a corona-proof protocol and the public who still prefer to avoid (socially distanced) crowds, especially indoors. My guess is that 'avoidance' is sustained by increasing evidence that Covid-19 transmissions take place via 'aerosols' that linger in the air, especially in indoor locations.
NL is one of the few European countries that has not (yet) mandated mask-wearing in certain situations. Instead, the NL govt. has left it up to local mayors to impose local mask-wearing where necessary.