A lot of this is because the name fits in with fashionable sounds of the moment (starts with a vowel, ends with -n.)
However, in the late 1980s there were actually TWO television shows in the USA that featured characters named
Ethan.
The TV western
Paradise featured
Ethan Allen Cord as its main character.
And
Ethan was the name of a child character on the popular 1980s TV series
thirtysomething.
The combination of those two shows is probably what caused the sharp jump between 1988 and 1989.