http://e55lzoju6enqswtvm66sk3uqukcin3377ehgiorplj4d7qyssipls4yd.onion/en/blog/why-bitcoin-mixing-is-important.php
This is because there's a risk of the mixing service turning out to be a scam and that it's just an elaborate way of stealing other people's cryptocurrencies. Of course, since only one server holds all the coins, making one huge transaction with so many individuals will risk overloading the network, making the process slow and therefore not cost-efficient.