Only because of voters. There has always been a bipartisan consensus in the House and Senate on increasing immigration[1][2]. The Republican representatives don’t want to lose their seats though and they know increasing immigration is the best possible way to get primaried[3]. Corporate Republicans want more labour[4], the same way Democrats do, without expecting that the immigrants will vote for them. Opposition to further amnesty has always come from below, not above.

[1] https://en.m.wikipedia.org/wiki/Gang_of_Eight_(immigration)

[2] https://www.msn.com/en-us/news/politics/last-minute-push-to-...

[3] https://www.washingtonpost.com/news/monkey-cage/wp/2015/02/2...

[4] https://www.hoover.org/research/republican-case-immigration-...

You're hard pressed to find a democrat who is actually earnestly pushing for immigration reform either. I'd claim the republicans largely swapped and the democrats are only for it just enough to foil the republicans!