Good question . As a native speaker I never learnt this as a rule so I can't really explain it according to any principles because they often seem flawed from a non-native's point of view. In the same way I also find flaws with the grammar of another language. Here is a link which gives a good, but not overlong, list of the use of subjunctive. You will see that 'remember that' is not included on the list.
I wonder if any academics out there can give a practical explanation.
ENGLISH PAGE - Subjunctive
PS I noticed that you corrected a subjunctive in another thread
(actually the original sentences with 'monitor' were fine as written. The word 'necessary' is similar in meaning to 'crucial', 'a good idea' in the list given in englishpage.com