A: Because “acronym” is often used to encompass initialisms as well, and it’s much better-known and more widely-used of the two words, despite it’s having come onto the linguistic scene later.
Merriam-Webster definition of ACRONYM
Merriam-Webster article about the two, which discusses the history, and some reasons why things get mooshy.

The word acronym is vastly more widely used:
According to Google’s Books Frequency NGRAM viewer, the frequency of the word ACRONYM reaches 0.000182% relatively recently, while initialism tops out at 0.00000323%.

Here they are together on the same graph. I almost thought the website wasn’t yielding the correct results, because the second line was so flat I didn’t notice it!