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