Why does Berwyn Heights and College Park have the SAME Zip Code?

Although Berwyn Heights and College Park are entirely seperate municipalities, the Town of Berwyn Heights is not large enough to support a full-time, full-size Post Office, so the Berwyn Heights Post Office (which is located in the Town Hall Building), is actually a "Sub-Station" of the College Park Post Office. For this reason, both locations have the same zip code.