The main reason is that 20% of all children still don't even get the most basic vaccinations - because they live in remote areas, in extreme poverty, don't even know about vaccinations or are just not being reached by the systems that are in place today. At the same time, together we can eradicate the once devastating disease of polio - once and for all.
The 100% campaign wants to make sure that all these unreached children receive these life-saving vaccines. The most important obstacles to reaching them are defective cold chains - required to keep the vaccines refrigerated until they really reach the child - and the lack of human capacity to reach these hardest-to-reach children: Trained health workers, reliable delivery staff, knowledgeable nurses.
Therefore the 100% campaign wants to raise funds to build these systems behind the vaccine - that the vaccine dose also reaches the child, and that the child actually gets immunized. While the actual dose often only costs cents, reaching the child and keeping track to ensure it gets all the necessary shots required to fully protect it costs around $25. These $25 really immunize the child and make sure it is protected against these deadly diseases - once and for all.