Why Didn't the Celtic Empire defeat the Roman Empire?

Because there never was a "Celtic Empire".  
The Millions of Celts in many dozens (perhaps hundreds) of individual Celtic tribes and groups were unable to conquer the highly organized Romans in a decisive war. As fiercely independent but disorganized individual tribes and groups of tribes they did invade and harass parts of the Roman Empire, and those actions ultimately helped bring it down.

But to have an Empire requires a centralized political organization and a centralized will like the Romans. The Celts never had that, and their offspring do not have it today. One of the defining features of Scots-Irish Celts (for example) throughout history is their inability to quit squabbling and get organized. The history of warring among the other Celtic tribes which make up much of Northern Europe is little better.

There was no Union of Celtic Republics, or United States of Celts. The individual Israelite tribes of Celts, and their Jewish Israelite cousins have always wanted to go their own way and do their own thing. From their earliest history as 12 brothers they have disliked each other intensely. Israelites have never been keen on joining all up under one big central government. It was true then, and it seems mostly true today.

Probably the closest thing to an "Israelite Empire" was the Kingdom of David circa 1,000 BC and that began breaking down soon after David's death. Persistent internal squabbling soon split the tribes into Northern and Southern Kingdoms and those tribes continued to fight among themselves, eventually resulting in their individual captivities.

Some scholars have fanciful visions of OT Israelites as peaceful 'hunter gatherers' living mostly off nuts and berries and dancing happily around a maypole like early hippies. It never happened. The Old Testament bears continual testimony to the fallacy of that vision and the grossly sinful and warlike nature of Israelites. Most of the sons of Israel wanted to kill their youngest brother Joseph out of envy. Reuben, the oldest, stepped in and saved him in a well to return to his father later. But Judah had the smart idea of selling him for money, so they did; for 20 pieces of Silver. The enmity between Judah and Joseph remains to this day.

The Northern Kingdom lasted in one form or another under Ephraim and his successors until taken into Assyrian captivity in 722 BC. The Southern Kingdom under Judah ended when it was taken into Babylonian Captivity in 587 BC. 

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