There are two key reasons which may explain this.
The first, is that EOS is a smaller platform than Ethereum, with a smaller market-cap, user base, and pool of developers.
Another possible reason is that DeFi users favor decentralization and security, over speed. dApps built on EOS tend to be those which are less dependent on decentralization and security, such as gambling games. Although Ethereum is slower, it provides more decentralization and security. This ensures that user funds in DeFi systems are as safe as possible.