...it would not be in our best interest to prosecute and didn't.
The case was prosecuted, and Microsoft was convicted (the conviction still stands). Microsoft appealed when it was found that Judge Jackson was giving closed-door interviews to reporters while the case was ongoing. Microsoft pushed an argument of unfair bias, and the appeals court bought it hook, line, and sinker. The findings of fact were upheld, though, but Jackson's breakup order was vacated.
Then Bush came into office and ordered the case to be closed. On the way out, Microsoft agreed to a consent decree where the Justice Department would oversee Microsoft's operations for a few years before Microsoft would be allowed to resume its monopolistic behavior.
Shortly afterward, Firefox started surging and Chrome appeared on the market. That made I.E. obsolete. Then Mozilla climbed all the way up to the top of the Stupid Tree, jumped off, and hit every branch on the way down. That allowed Chrome to take over.