Smaller commercial airliner maybe, some countries do use A320s and 737 as head of state aircraft. Business jets are just too small. Think of all the vehicles in a motorcade — protection/security, communications, health/medical teams, etc. — all filled with people. You maybe have other high level positions with their crews as well. You’ve also got press and news crews following along. All of those people have to get on the plane. A business jet with 20 seats won’t cut it.