I think the question would more correctly be posed as "why did the United States, decide to celebrate Pi Day on Albert Einstein's birthday?"
Pi Day was started by Larry Shaw in 1988.
Einstein was born in 1879 and died in 1955. (He was already dead when Pi day came into existence, so there is no way he could have been born on Pi day.)
See the related links below for additional information.
God has a sense of Humor.
Albert Einstein was born on Friday 14th March 1879
Albert Einstein
Albert Einstein was born in Ulm, Württemberg, German Empire (present day Germany).
Albert Einstein was born 14mar1879 in Ulm, Germany.
albert einstein
Albert Einstein
Albert Einstein died in 1955.
It was Albert Einstein who was born on Pi Day of March 14th
He was born on March the 14th on a Friday in 1879.
He was born March 14, 1879
Albert Einstein was born on Pi Day which was March 14th 1879