Because his time is running out. He desperately wants to get the Security and Prosperity Partnership of North America (SPP) put in place before he leaves office. And a large part of that is, in effect, eliminating the border between the US and Mexico. I guess he thinks that will be a great legacy.
If you'd like to read what's in store for us, follow this link, download the .pdf file and begin reading on page 23 which is page one of the report.
‘Roger’ that!