This is required to meet the increasing costs of organizing this event. Some costs may not be visible to a participant, like costs associated with having a team of developers for building the 100 Days of Running platform and improving it year after year. Designers to do the creative work. Support staff to attend to all support requirements from the participants. Costs associated with the infrastructure on which the platform runs and which needs to handle the ever increasing load. And this support is required for an extended period starting from Jan all the way to almost September, a period of 9 months.
All these requirements continue to grow as the number of participants increase, and we have basically reached a point where it would just not be feasible to execute this event without charging participants for the same.