When you have two currencies (say USD and BTC) and one goes down in value every year while the other appreciates at a very high pace, you have little incentives for spending the good one.

It's still crucial for Bitcoin's success to be adopted as a medium of exchange and not just a store of value. As it grows in adoption and reaches more stable prices, there will be more incentives to use it. Specially for people that are already getting paid in Bitcoin.

Apart from that, the incentives to spend Bitcoin already are: cheaper processing fees for micropayments (LN vs. Visa), privacy, banned items or jurisdictions, ...