It is a much older implementation that works on Bitcoin's UTXO model (rather than the account model in Ethereum) and without smart contracts. It doesn't have a solution to the nothing at stake long-term attack, but it thwarts all known short-term attacks. Personally, CASPER's solution to the nothing at stake problem is concerning. In an ideal world, it is ideal, but in a more practical world, I can definitely foresee someone doing something wrong or making a mistake (either developer bugs, or consumer running two wallets, etc), unintentionally making a block on another chain, and losing their $1M worth of ETH as a result. It only takes that happening once, maybe twice, to get people to think twice about staking, and when less people are staking with less coins, the network is much easier to attack
(disclaimer: I work on a blockchain project that is somewhat a competitor to Ethereum)