Why were you surprised that Republicans were responsible for the passage of a bill that the business community desperately wanted and labor unions fiercely opposed?
Republicans tend to be more nationalistic and anti-immigration, both of which tend to go with protectionism. Also, which party's convention did Pat Buchanan speak at?
Buchanan got 20% of the vote though, so it's not like all Republicans in the 90s were raging paleoconservatives. Pre-Trump Republican voters were much more pro-trade than Democratic voters, though the gap narrowed during the Obama years.