No one has "allowed it". You're lying. Trump is the guy who opened up the Southern Border. Illegal border crossings were at a 50 year low when Trump took office, and stopped prosecuting employers.
Under the Trump presidency, there has been a dramatic shift in arrests of undocumented immigrants from the Southwest border to the interior of the nation.
www.npr.org
Within 2 years Trump was declaring a National Emergency at the Southern Border.
Democrats have called the declaration "unlawful."
www.texastribune.org
Trump broke it and now he blames Democrats for "open borders". While Biden, once again, has fixed the mess that Donald Trump made: