I guess annoussa doesn't need this explanation anymore, but probably others might get some help.
I second Susan's and Mimino"s replies, adding that the connector adverb "so that" implies a cause and a result:
"The blind man didn't cross the street because he couldn't" - cause (and both verbs are in the past).
"The policeman stopped the traffic" - result (also here the verb is in the past).
When you join both cause
with the connector "so that", the modal "can" is implied; the situation which described a disability occurred in the past, therefore the modal will be also in the past: "could".