MGM chose to settle. The NYT reports their legal defense as being this:

It had sought to block victims from recovering any money from the company, arguing that a little-known federal law passed in the aftermath of the Sept. 11 attacks meant that MGM enjoyed a shield from liability because the shooting qualified as an “act of terrorism” under the law’s expansive definitions.

Because of that — and also because a security firm hired for the concert possessed a special designation from the Department of Homeland Security — MGM argued that its interpretation of the law meant that it should not have to pay damage claims to injured concertgoers. The federal law is known as the Support Antiterrorism by Fostering Effective Technologies Act, or Safety Act.

However, MGM may have found settling preferable to the negative publicity that would be incurred by legal victories in the court system, as cases would drag on for years as each victim (or relative) recounted the effects of the shooting on them. Their decision was likely informed by this:

As part of its strategy to have the company declared immune from liability by funneling cases to a federal court where that issue could be litigated, MGM sued more than 1,000 people who had already filed cases or indicated an intent to pursue claims against the company. Though the company’s lawsuits did not seek any money, the strategy stirred anger against MGM.

It sounds like, although I am not sure, that MGM was suing them so that a federal court could handle the issue in its entirety rather than having to go through 1,000+ cases (i.e. similar to a class action law suit). The blowback from that strategy meant they could either continue with that strategy despite the public blowback, litigate all the cases individually, or reach a settlement. They chose the last option.