Same reason Ronald Reagan granted 3 million of them amnesty. He stated that he had just handed the Republican Party 3 million new voters.
George W Bush wanted to do is same for an additional 10 million. He also said he'd be handing the Republican Party 10 million new voters.
That would be an additional benefit with Democrats, but they see it in terms of what they already have, millions of foreign born new citizens who would greatly appreciate seeing more of the very people they left their countries to get away from be allowed in.
Like Republicans, Democrats don't see people, they see lumped categories and think that for some reason Puerto Ricans would want to see millions of other Latin Americans coming to the United States. They don't see individual people voting individually. They don't even see the fact that even if lumped as places of origins, Hispanics from one country tend to not like Hispanics from other countries.
It's like people from the US, Canada, Guyana, Belize, the Falklands... We're all not exactly identifying with each other nor seeing each other as being part of one group just because English is our primary language. But while we're acting like North and South and Red and Blue and blah blah blah are completely genetically incompatible different species, we think from the Bronx all the way to Tierra del Fuego anyone with any Spanish in their background is just a bunch of cuddles.