As the schema guide explains, in 1999 they picked XML based on the study from 1996:
"Following a 1999 feasibility study on XML/SGML, the Committee on House Administration adopted XML as a data standard for the exchange of legislative documents" http://uscodebeta.house.gov/download/resources/USLM-User-Gui...
It took the US Govt 17 years to release 200,000 pages of the US code in XML.