Glad to have you on board!
I’m sure Mr. G or about anyone else here on the site can give you a better answer (I know very little about music theory), but I’ll try to answer this best I can:
I believe the reasons for using flats over sharps at times is to keep each note in a particular key a different character in the alphabet. The key of F is: F G A Bb C D E F. If you made the Bb an A#, it would look like this: F G A A# C D E F. See what I mean, no B and two A’s? Here’s a better example: The dreaded key of B is spelled as such: B C# D# E F# G# A# B (five sharps). If we used flats instead of sharps on this key, here’s what we have: B Dd Eb E Gb Ab Bb B (no C or F and two consecutive B’s). This is much more confusing than keeping each character unique whether it’s flat or sharp.
I think it may be based on the key of C: C D E F G A B C (no sharps or flats but each letter is different).
Maybe you already know this, but if you notice in most Hymnals, each song will have a number of flats or sharps in the top left. This is the sign for the specific key the song is in. One sharp would be the key of G… one flat would be the key of F… Five sharps would be the key of B and so on. If there’s nothing there, it’s in the key of C.
Ppsshewwwww, Sure hope I’m right about this and hope it helps!