Minecraft wiki entry for Caves_%26_Cliffs, associated text: Most of the overworld ores have been retextured and touched up mainly for accessibility reasons. These also apply to the new deepslate ores.
This is a great help for color blind people that have difficulties distinguishing a certain ore of similar texture from another.
Diamond ore is an exception to this for being the "iconic" ore of the game, making Minecraft developers want to keep the classic feel of it.
Ore distribution has been changed for more strategic mining.
Certain ores are more common at certain depths, and some may generate in lower quantities or not generate at that level at all.