White campion

I don't know what happened in this particular case, but there are two common reasons: (1) The plants that were once considered separate species are found to hybridize so frequently that they can't be considered separate species [plants that hybridize only occasionally in the wild, e.g., members of the red oak group, are still considered separate species]; (2) Molecular analysis, particularly the sequence of one or more ribosomal RNA genes, shows that the plants are genetically equivalent. Once the species are combined, it is usually takes the name of whichever species was named first.

Do you know why they decided to merge the species?

Silene latifolia now includes what used to be two separate species, bladder campion and white campion. The downy stem and calyx, and relatively uninflated calyx, is typical of what used to be called S. alba.