The borrow and supply rates for stablecoins are much higher compared to WBTC and WETH because of the utilization ratio. Utilization ratio is the ratio of borrow asset over deposited asset. Stablecoins are much more popular lending asset as it can be used in many cases hence higher demand for them compared to WETH and WBTC.

It is almost the same pattern for the daily variable rate except it happened at a bit lower value.

From the two graphs below, we can see the daily variable and stable borrows rate of the 5 assets on AAVE V2. The stable rate for the three stablecoins USDC, USDT and DAI are quite erratic from early March to end of May then they reached a plateau from then till early August. During that period, WETH and WBTC remained at constant rate ranging from 3 to 6% while overtaking each other.