Availability of magic
If your setting is a High Magic setting (see pages 38-41 in the Dungeon Master’s Guide for more on “Flavors of Fantasy”), this might not apply so much, but in settings where magic is not quite as common, not everyone has access to a spellcaster who can cast sending, so then more people have to rely on more mundane ways of sending messages.
Even in a High Magic setting, or in a lower magic setting where you happen to know a spellcaster, having a spellcaster cast spells for you often costs something, and not everyone is able or willing to do that when sending a messenger is likely to be cheaper. Consider the AL rules on clerical services as an example of putting prices on spellcasting.
The spell’s description says:
You send a short message of twenty-five words or less to a creature with which you are familiar.
If you needed to send a message that was longer than 25 words, you’d need to find another way, so a messenger with a piece of paper (or several), or even just having the messenger know of the subject (an apprentice delivering a message for their master, no need for paper since the apprentice understands what is being discussed).