Why do some cities end in burg and some in burgh?

Answer 1

While a "burg" may or may not be on a river, a "burgh" is supposed to always be where one or more rivers converge.

"In 1890, the United States Board on Geographic Names decided that the final h was to be dropped in the names of all cities and towns ending in burgh. (Throughout the period 1890-1911 city ordinances and council minutes retained the h.) In 1911, after protest from citizens who wished to preserve the historic spelling, the United States Board on Geographic Names reversed its decision and restored the h to Pittsburgh."

Ref:

http://www.carnegielibrary.org/exhibit/hname.html

http://www.profootballresearchers.org/Coffin_Corner/27-05-1091.pdf

Answer 2

Both "burg" and "burgh" are used to denote a fortified town or settlement in old English. The use of "burg" is more common in American English, while "burgh" is more common in British English. The spelling difference is largely due to historical variations in Old English and Scottish Gaelic influences on place names.