I suppose there's not much hope of finding out why one baby was vaccinated and the other wasn't? Was it cost?
Hmm, I doubt it. The photograph was taken in the US, and the public vaccination campaigns (smallpox, polio, DTaP etc) by and large have been basically subsidized programs. While cost is a continuing factor in some developing nations, developed nations generally have a significant subsidy program for at-risk groups or disadvantaged populations.
The WHO Global Vaccine Action Plan, UNICEF, and the Gates Foundation have been pushing the "Decade of Vaccines" between 2010 and 2020, and those three groups (as well as GAVI) provide most of the vaccine coverage to the developing world these days.
In all likelihood, the mother of that unvaccinated kid was lower-class and had no time to bring a child to a vaccination center. For many of those kids, public schools were the first time they were vaccinated, so even though the resources were out there, it was not until they were absolutely mandated to be covered that they were.
When people had to choose between working to feed their children that week by taking up extra jobs on their one day off, or possibly losing a child down the line, the brain naturally thinks about what's more imminent. It's a no-brainer to put off vaccination until they get them in school, or at least it probably seemed that way...