Because people typically think in terms of contiguous addresses being "horizontal," though I have no idea why.
Lower memory addresses are often considered to be "below" or "left of" higher addresses. Yet when talking about a stack, the "top" of the stack is at the lowest memory address (usually). I just think the term "row-major" brings in too many assumptions, even though that's the term people use.
"The addresses in the array vary the slowest along the last dimension" might be a better way to put it.