According to: Air Force Times Article

Some highlights on why:

■ A number of pistols can outperform the M9, which is manufactured by Beretta. There have been significant advances in trigger mechanisms in the 26 years since the M9 entered service. The Army’s Small Arms Branch at Fort Benning, Ga., put these to the test and found troops had more hits and tighter groups of hits as a result.

■ There are too many issues to overcome. The M9’s slide-mounted safety is one issue. When troops rack the slide to alleviate a jam or stovepipe, they often inadvertently engage the safety — and won’t realize this until they reacquire and squeeze the trigger. The open-slide design allows contaminants and dirt into the system.

The 9mm round also lacks the stopping power most soldiers need. And an improved M9 would need a modular grip, integrated rail, night-sight capabilities and the ability to suppress fire.

■ The numbers don’t add up. Easlick’s team did the budget drills and found that a new pistol would be less expensive to produce and maintain.

How much cheaper? Simply improving the M9 would be a “waste of time and money,” Easlick said.

For example, many newer pistols use a polymer frame. While that will not be a requirement, that frame is cheaper and more durable.

■ User feedback. Unlike the M4 carbine, which has a strong approval rating among users, the M9 consistently ranks as the weapon in which troops have the least confidence.

Another interesting tidbit was that soldiers that were issued the entry-level S&W (tm) Sigma (tm) actually preferred them to the M9. The M&P (tm), as well as others, should perform very well in any trials then.