This is the reason given in Australia by Parks Victoria

LEAVE NO TRACE

Advocate for minimal-impact practices wherever you go. Many people are surprised to find no bins in national parks. Waste attracts native animals, which can change their natural behaviour and harm both natural and cultural sites, as well as your personal belongings.

Always bring rubbish bags (and one for your neighbour) and take all your rubbish home. Help educate others about the importance of leaving the park pristine, minimising your impact on the delicate balance of the ecosystem.