Fighting capabilities: US vs German infantry at Anzio

What were the primary deficiencies in US infantry divisions in early 1944 and why did this battle not end in a paralyzing German tactical and operational defeat?

Operation Fischfang-German Infantry counterattack against the Anzio beachhead

I find this combat action to be an interesting case study of US vs. German infantry organization and fighting capabilities on a more even scale.

For one, the Germans aren't hopelessly outnumbered like they were in Normandy. Although the early 1944 German infantry units were much lower quality compared to those that served in 1941-1942, the units deployed against US forces at Anzio were mediocre but dependable.

This battle resembled WW1 warfare more than WW2 fighting. The terrain was too muddy for Panzers to provide close support and the German infantry essentially went it alone, attacking US defense systems with small arms across open, flat treeless fields. The Panzers supported the German infantry from long range and pummeled US defenses with shellfire.
The German assaults ultimately ended in failure. Only shallow penetrations were achieved and US artillery superiority played a decisive role in shattering the attacking German infantry units.

Order of battle

Elements of 2 Panzer divisions and 7 German infantry divisions attacking 5 US , 2 British infantry divisions and one US Armored division.

432 Allied artillery pieces vs. 452 German artillery pieces but Allied Artillery fired 15-20 times more ammunition.

Allied Air supremacy-> 550 light/medium/heavy bombers w/200 fighter bombers

Final tally:
5,400 German infantry losses (including 600 German captured) vs. 3,500 Allied infantry losses

-these are combat losses only

-German commander states that 75% of his casualties were caused by artillery fragments

-This concludes that 1,350 German infantrymen were probably injured, captured, or killed in firefights

-Why were US infantry combat casualties so high regardless of US fire superiority and defensive advantage?

 

1. Much easier to defend Italy than to invade.
2. The US commander made the mistake of sitting put allowing the Germans to consolidate
3. The US forces were packed in the Anzio beach head and made easy targets.

 

Take into account also that German Supplies were also limited (look at the Artillery Stats, for instance - Germans would have run out firing that many shells) not to mention Allied Air Superiority, which aides in spotting for Artillery and monitoring enemy movement, along with Interdiction.