Because President Reagan signed the first, and so-far only, genuine blanket amnesty for illegal immigrants in the nation's history. Like it or not, avoid the truth or not, but the 1986 act was a sweeping amnesty and those who benefited from it paid no penalty at all for crossing the border illegally. It matters not that the numbers of people who received amnesty were fewer than today (btw, about 3 million, not the 1 million you stated). In addition, people across the political spectrum acknowlede that the 1986 act merely opened the floodgates and caused the situation we have today.
Nobody is avoiding the truth, we know he signed the Mazzoli/Simpson amnesty. In fact unlike those today peddling "pathway to citizenship" he called it an amnesty. Most of us agree it was a mistake not to be repeated. I'm not sure exactly what your point is other than because it happened then we should it again.
(btw, about 3 million, not the 1 million you stated).
It ended up being 3 million after the fraud that even Chuck Schumer admitted had occurred. The projection was that it would only be a million or less.