Why is Solana not decentralized?

I've been hearing a lot about Solana, and it's touted as a high-performance blockchain. However, I've also come across discussions questioning its decentralization. Could you explain why Solana is sometimes criticized for not being fully decentralized? I'm interested in understanding how its architecture or governance model might contribute to this perception. Also, are there any specific instances or events that have sparked these discussions about Solana's decentralization? It seems like a crucial aspect to consider when evaluating a blockchain project.