https://www.brookings.edu/articles/the-collapse-of-bipartisan-immigration-reform-a-guide-for-the-perplexed/
The easiest explanation is that Republicans in both the House and Senate yielded to objections from their all-but-certain presidential nominee, former president Donald Trump. Once the House Speaker stated publicly that he would not allow the Senate bill to reach the House floor for a vote, Republican senators were unwilling to run the political risk of supporting a measure that would not become law.
However, there are deeper reasons for the deadlock over immigration. The last comprehensive immigration reform was enacted almost four decades ago, during Ronald Reagan’s presidency. This bill represented a grand bargain between elected officials who sought to extend legal protection to millions of migrants who had entered the U.S. illegally and officials who were most concerned about stemming the flow of such migrants. The bill accomplished the former but had no discernible impact on the latter, leading many conservatives to denounce it as an “amnesty” bill.