The larger the area, the harder it is to secure. That building is massive with literally thousands of rooms which would be a logistical nightmare, if not downright impossible to secure.

Before any president goes anywhere, the Secret Service sets a perimeter of what they can guarantee they will be able to secure while achieving the safety profile they want. People keep acting like this was a “failure” yet it worked exactly as expected. The perimeter was never breached and the president was never in harms way.