Why is Iron Ore so rare at Y Coordinate equal to 5?

y=5 is extremely inefficient for mining pretty much everything. Generally, the lowest you want to mine is around y12 if you're looking for a number of things like diamonds, Redstone and so on.

To answer the question in the title, the reason why Iron is so bad to mine at y5 is that this the point that bedrock starts to appear as well as being within the range of lava. From the below graph you'll see that around y5 everything begins to drop off and the rarity of ores increases.

Nothing has actually changed in terms of ore generation and rarity with 1.15.2.

From a statistical standpoint, you can see the likelihood of ore from the below graph along with additional information to specific ores.