There are plenty of ores on starter islands, I don't think more need to be added. Since ores are consistent no matter the mode, mining diamonds, say, in basic mode is quite overpowered considering the best armor one can get is chain. Same thing goes for iron in the form of iron armor and gold in the form of golden apples.
I also think the main gameplay of skywars is focused around the chest loot rather than the blocks on the map. Ores can be useful, but I think they are better as a supplement to gameplay rather than a centralizing feature.