What could be a possible reason for such requirements?
It's not only here, but a lot of websites won't let you set a password longer than 16 characters, or won't tell you about it and will just cut it.
tfw when you can't use your all emoji password on most websites
It makes sense to have some limit; otherwise you could create a few accounts each with multi-gigabyte-length passwords if you so chose.
Why 15? User negligence.
Thanks for listening. Have some Jeb! vines: