If I'm long the index and believe they will go up long term, why would it be a bad idea to buy SSO and QLD that are 2x the index?

If I'm long the index and believe they will go up long term, why would it be a bad idea to buy SSO and QLD that are 2x the index?

I understand there's volatility decay. The 2x index seems to capture the majority of the upside. For example, the S&P 500 5 year total return is \~70%. SSO 5 year total return is \~116%.