This may answer your question:
"
QUESTION: Why do
ser and
ir have the same form in the preterite?
First, it is really the perfect (= Spanish preterite) conjugation of the Latin verb ESSE "to be" which came to be used for both
ser and
ir in Spanish. In Spoken Latin, the preposition IN (> Spanish
en) used with a location came to mean "movement toward". Also,
ser and
estar did not have the same restrictions in Old Spanish that they do in modern Spanish and consequently
ser occurred at times where one might expect
estar. Thus FUERUNT IN CAMPUM, for example, originally meant "they were at the countryside", but later "they were moving/in route toward the countryside", and eventually came to mean "they went to the countryside". Therefore, this paradigm,
fui,
fuiste, etc., eventually became associated with the infinitive
ir "to go"."
http://www.virginia.edu/cla/avd/doc/...h_history.html