>>448095406A lot of factors, actually!
One, they were a large target, armed to the teeth, sure, but it made it easy for small hit and run attacks against villages and towns because they couldn't make it in time unless troops were already there.
Second, overexpansion. Their armies were fine until they started invading Britain and eastern europe, they had to start forced recruiting, which didn't earn them any favors.
third, They had a slave based economy, and 10,000 slaves died annually, which meant they had to be constantly fighting. never ending war, just to keep afloat.
On top of that, social instability, they had the radical christian sect within their own society, much like what would happen if the US officially occupied the middle east, except with islam.
Eventually they had to resort to paying people large sums of gold per year to NOT attack them.
Overexpansion, oppressive rule to stay in power, social decay and their armies were too spread out to be effective.