Because they are not mitzvos temidious. You don't have to perpetually have them on your mind.  Once a day is sufficient.

Actually, I am not sure that all authorities require a daily remembrance. It may be only al pi kabbalah that we do it daily. Gotta research that point. Clearly zechiras amalek is done b'eztzem only once a year to satisfy the shikcha requirement of 12 months...

If you work real hard you can see a connection between the six daily remembrances to the six constant mitzvos. One or two are a stretch...