Why did stone castles replace the motte and bailey castles?

Timber, one of the two materials from which motte and bailey castles were built (the other being earth), was perishable (rots) and, more importantly, vulnerable to fire. A more durable and resistant medium was required and that was provided by stone.

Compared to the motte and bailey, stone castles were larger taller and more reliable for defence purposes. They gave better protection against attack, fire and cold rainy weather.

Why did they stop building castles?

Castles were great defences against the enemy. However, when gunpowder was invented the castles stopped being an effective form of defence.

By the end of the 1300s gunpowder was widely in use. The medieval castle with its high vertical walls was no longer the invincible fortification it had been.

Why was gunpowder so effective?

The use of gunpowder made both castles and city walls much more vulnerable because a cannon could knock down the stone walls. The only way to knock down walls before the invention of gunpowder was to dig under the walls to make them unstable. This was a highly dangerous way to do it especially as you might get boiling hot tar poured over you from above!