Because SoftBank wants additional control of the company. In order to get that, they need more stock. Stock than Neumann owns. So they'll pay him for it.
They could also buy the existing debt from current lenders then wait for the inevitable restructuring before putting more money in and getting equity out of that without having to buy it from Neumann. They seem to have rejected that approach given this news but it would be interesting to know why - is it because they think it would damage the business (tenants leaving etc in reaction to a restructuring/insolvency) or because they were worried about the reputational damage to Softbank/Vision Fund?