Sequelize is primarily an ORM. I won't get into a religious debate, but there are many good reasons for not wanting to use an ORM.

Slonik is much newer, and is Postgres only. But there is a general trend seen in many different projects that support safe SQL execution using tagged template literals in JavaScript, which is a fantastic technology. Slonik was one of the first projects that went this route, but there are now many others.

In general I think it's a mistake to look for a single "market leader" for node DB access. There are many high quality libraries that are favored by different teams for various reasons.