Part of snoring is low frequency sounds, which even with fully plugged ears, transmits through bone conduction.

also long term use of ear plugs with sleeping can cause unwanted side-effects.

When we sleep our hearing system does not turn off (safety), and when you wear ear plugs while sleeping you run the risk of making your hearing work harder - and then if you have no plugs in your hearing system is now (hopefully temporarily) more sensitive than before.