I was told by a local that, mirroring what other people are saying, it’s so the menu is easier to read and readable by more people.

In the past, Japanese literacy was lower and immigrants also had difficulty reading Kanji (especially Chinese immigrant, since Japanese Kanji is often altered in form and sound when referring to the same thing).

Japanese restaurants mostly keep doing it out of tradition rather than necessity.

(Running counter to this, some Japanese restaurants also use the outdated Yamato style of writing for traditional and aesthetical reasons. Even locals sometimes have trouble reading those character and its use is usually relegated to the shop’s name.)