Even though Congress passed antitrust legislation, why did corporate leader try to retain the concept of trust?

It's a question of scale and market dominance. A trust can be any size, from personal to international conglomerate. They are generally beneficial things. The problem is when they get too big, especially relative to their industry, they become monopolistic and anti-competitive. The anti-trust legislation passed was intended to address these problems without compromising the benefits they offer to smaller investors.