The exact wording:
Level 1: Martial Arts
Your practice of martial arts gives you mastery of combat styles that use your Unarmed Strike and Monk weapons, which are the following:

• Simple Melee weapons
• Martial Melee weapons that have the Light property

You gain the following benefits while you are unarmed or wielding only Monk weapons and you aren’t wearing armor or wielding a Shield.
Bonus Unarmed Strike. You can make an Unarmed Strike as a Bonus Action.
Martial Arts Die. You can roll 1d6 in place of the normal damage of your Unarmed Strike or Monk weapons. This die changes as you gain Monk levels, as shown in the Martial Arts column of the Monk Features table.
Dexterous Attacks. You can use your Dexterity modifier instead of your Strength modifier for the attack and damage rolls of your Unarmed Strikes and Monk weapons. In addition, when you use the Grapple or Shove option of your Unarmed Strike, you can use your Dexterity modifier instead of your Strength modifier to determine the save DC.

Why can't I choose to use my Strength Modifier instead of my Dexterity Modifier for Martial Arts Attacks? The wording specifically says "may" not "must". This makes no sense.

The D&D Beyond setup seems to automatically pick the better option when two different calculations are in play.  If the Strength Modifier is higher for that character on that  particular weapon, it will use that one. If the Dexterity modifier is higher, it will use that one.

I'm guessing the decision was made that "Situations where players will want to use the lesser modifier will be very rare, and thus manually handled"

The only one I can think of offhand is a Raging Monk/Barbarian, for whom STR Modifier < DEX Modifier < STR Modifier + Rage Damage Bonus, and it looks like D&D beyond automatically applies the Dex Modifier to the Attack Roll, and the STR Modifier + Rage Damage to the damage roll, which is probably not the way that should work, because you need to be attacking with Strength to get the Rage Damage bonus.