Why is it inappropriate to leave the Blessed Sacrament alone when exposed in a monstrance (e.g. Adoration Chapels must have at least one adorer at all times), but appropriate to do so when it is replaced in the Tabernacle? What is so special about the walls of the Tabernacle?
That’s an odd why to put it.
First, the tabernacle can be locked. It is like a safe. A person with bad intentions cannot easily get into a tabernacle for the purpose of theft or desecration of the Eucharist.
That is not the case when the Blessed Sacrament is exposed in a monstrance.
This is also a good opportunity to point out that tabernacles should be made from the proper materials. They should be strong and safe as well as dignified and beautiful. Normal glass, for example, is not an appropriate material.