I see two points why this could be the case, but they are only based on my intuition.
The first one is that the distribution of coins is not decentralized any more. In a PoS no new coins are generated during the creation of a block. Hence all coins have to be distributed via one or more central authorities. By switching to PoS at a later point in time a rather large amount of ether coins has already been distributed among all participants of this blockchain.
The second reason is the amount of influence one party with a rather large share of all ether coins can have a significant impact on those transactions that can enter the blockchain. In PoS those with the most coins have a higher influence on deciding which transactions are to be minded, compared to Proof of Work (PoW) where the computational power is the key factor (again: PoS ONLY requires a lot of ether coins, no computational power). This could allow a wealthy party that owns many coins to prioritize certain transactions and never include (and thus potentially excluding) specific transactions.
I see these two problems with a PoS beeing used in a blockchain upon the very beginning of it, espcecially because they accumulate each other.
I hope that helps you a lot. Feel free to correct me if I am mistaken or bring up more reasons.