The answer is the left has a long history of opposing globalization, union breaking and neoliberalism in general. In particular the left attacks on centrist democrats embrace of neoliberalism has be vicious. They were the reason why Obama won the primary over Clinton in 2008. And why Sanders showed well in 2016 even though Clinton won. And Clinton despises the left for it.
See the 1999 Seattle WTO protests as an example of the left opposing neoliberalism.
You say "the left" like it is a well-defined thing or a single entity.
Just because those that were further-left than centrist democrats existed and opposed their centrist democrats policies doesn't mean centrist democrats weren't considered "on the left" by a great many people, who would also be considered left of the Republicans at the time.
See the 1999 Seattle WTO protests as an example of the left opposing neoliberalism.
https://en.wikipedia.org/wiki/1999_Seattle_WTO_protests