The difference, Jill, is that at Adoration the host is in a monstrance, exposed to view for a specific purpose: adoration/devotion. If there is no one attending, the devotion should not be "taking place," because in fact it is not occurring. This is uncanonical negligence. The repose of the Sacrament in the tabernacle is just that: repose. This is why the tabernacle is closed and locked.
In real life, there are necessary exceptions to the rule of never leaving the Lord alone. In my locale, there is only one 24/7 perpetual adoration chapel. A few other parishes have the adoration devotion, but only for a few hours each week or month. I belong to the 24/7 group, and for most of the night hours (mine is pre-dawn) there is only one adorer assigned. In this situation, if a person gets sick or needs to go to the bathroom, he does what he has to do, returning as and when he can.
I've heard that He can only be reposed 2 times in 24 hours. Why is that?
I am not familiar with this rule. Presumably it has to do with excessive handling of our Lord.
I am aware of a few isolated chapels where the Blessed Sacrament resides in a monstrance within a tabernacle. A person can obtain the key from the office, open the door to adore and close it before leaving if no one else is there. If several people come in at different, non-overlapping times, the door can be opened and closed several times in the course of a day. But here, the host and the monstrance are never touched, so it is a different circumstance.