The first was that the 1911s then in service were wearing out, the last batch having been purchased shortly after WWII.
Second was ammo compatibility with NATO countries, as the STANAGs (standardization agreements) required.
Third was a desire for a lighter-recoiling pistol for troops who are/were inexperienced with pistols, who complained that the 1911 kicked too hard.
Fourth was a desire for a pistol more reliable than the 1911. The 1911s used for comparison in the JSSAP trials were markedly less reliable than the modern pistols entered in the competition. IIRC, the 1911s had something like 500 mean rounds before failure, while the modern pistols were well into the thousands or tens of thousands.
Fifth was a desire for a DA pistol with a decocker, as the 1911 was found to be less safe in the hands of inexperienced troops, leading to many NDs.
Sixth was a desire for a high-capacity pistol, to give inexperienced troops more chances to hit the enemy.
Neither the HK45 nor the Mk 23 nor the USP existed during the original JSSAP trials. At the time, I believe the only HK .45 pistol available was the P9S, which met few of the JSSAP criteria.
All that said, a reliable 1911 is a very fine fighting pistol. It's just that the chances of getting a reliable one are considerably less than getting a reliable modern pistol.