My first bitcoin buy from Local Bitcoin and it was my favorite p2p site. But I don't understand why and what is the main reason to closed buy/sell service by local bitcoin?

It is since February and now you talked about it. You are too late to realize about their service closure.
LocalBitcoins Exchange Closed After 10 Years of Operation.
It is not only LocalBitcoins, there are some P2P platforms closed their services too like Paxful, 2 months after LocalBitcoins.
Paxful is shutting downThey did it by stricter regulations. Even Kucoin required mandatory KYC a few months after that.
Before LocalBitcoins and Paxful, there is another one in October 2022, Localcryptos.
LocalCryptos is shutting down