Because:
1. Fast BFT. It's easy to implement dPoS on top of it.
2. Creators of Tendermint are building the Cosmos project - interblockchain communication (transferring if tokens).
3. It's easy to implement a custom transaction logic for your dapp/blockchain by subscribing to Tendermint messages.
4. Over 50 projects are building on Tendermint. Some of them:
4.1. Hyperledger Burrow - PoS imlementation of Ethereum. Hyperledger Sawtooth uses Burrow to enable smart contracts.
4.2. Sentinel dVPN - decentealized VPN is built on Tendermint and you can download their app from Google Play and use.
4.3.
Altcoin.io - DEX uses Tendermint as a sidechain paired with their Plasma smart contract on Ethereum