It may correspond to availability of the satellite to take the photos of the area.
First the satellite has to be over the area. Second the satellite has to be free from other tasks to take photos of the area. From Wikipedia, the satellite's mission involves:
Proba-V will support applications such as land use, worldwide vegetation classification, crop monitoring, famine prediction, food security, disaster monitoring and biosphere studies.
Taking photos of a single lake in Bolivia may not be high on that list of priorities.
Interesting speculations, but that's not how these environmental monitoring satellites work. They are set up to get global coverage with this instrument every 5 days. The orbit is chosen so that this happens, to enable long-term time series of vegetation, land cover, etc., as you note.
It's a successor to other satellites like SPOT, and a competitor/complement to NASA satellites like MODIS.
First the satellite has to be over the area. Second the satellite has to be free from other tasks to take photos of the area. From Wikipedia, the satellite's mission involves:
Proba-V will support applications such as land use, worldwide vegetation classification, crop monitoring, famine prediction, food security, disaster monitoring and biosphere studies.
Taking photos of a single lake in Bolivia may not be high on that list of priorities.