The simple answer is that it is hard to make. Everything comes with a price, and the price of using the ultimate biocide is that you have to make it where you use it or suffer the consequences of trying to move it. Chlorine Dioxide has been made by mixing concentrated chemicals and by using electricity. Both come with hazards, as described in the Fig. 3 chart.
Once it is made, however, storage of Chlorine Dioxide is a real issue. UV light, heat, and time all degrade it, and the dissolved gas does not want to stay in solution. In addition, these problems are more severe in concentrated solutions than they are in dilute solutions. Although it is possible to store Chlorine Dioxide safely at normal temperature for short periods of time, the storage solutions require additional engineering and cost.
These reasons are why the CLO2IX® systems have been engineered to dispense an inherently safe 700 mg/l solution on demand without the need for storage.