Note that Byzantine grenades were more like Molotov cocktails than what we think of as grenades.

Main reason why grenades were used in a very limited fashion until the early modern era (16th century) is the lack of a reliable fuze. It wasn't until the gunpowder fuze that you could reliably light an enclosed and thrown explosive charge, early gunpowder weapons (outside China and India) used loose gunpowder to guide the flame to the main charge and that loose gunpowder would have been scattered when you threw the weapon.

We see the first traces of match cords In the mid 15th century