There is a social ladder. Cool kids on the top, unpopular kids at the bottom. You and I are probably in the middle.
New people begin near the bottom of this social ladder. They’re nobodies (which is slightly better than being unpopular). These nobodies feel like outsiders. They’re effectively walking into a big room of people who know each other. It’s scary. It’s easier to lurk than risk participating.
You need to jump-start newcomers up your community’s social ladder. Give them a way to feel popular and wanted amongst the group. A good welcome helps. Mention them in a newspost with other members. Introduce them to other members with similar interests or who live nearby. Do the hard work of introducing themselves for them.
Also make them feel appreciated. Ask their opinion on topical issues in the community. Have a newcomer-only category in your forum. Consider having an 'Ask a veteran' category too.
The higher members climb the social ladder, the more they participate. But, if the social ladder feels too steep in the first place, most newcomers wont try to climb it. You need members to feel they can be popular in the community without having been a lifetime member. It’s not easy, you just need to provide the right jump-starts.