Why did the men, who wrote the Constitution, permit and protect the enslavement of Blacks?

Some of the founding fathers thought that blacks were innately inferior to whites and they could never be treated as equal citizens. If they were emancipated there would be a sizable population of people who weren't full citizens and yet couldn't be controlled as easily as slaves could. A combustible situation. Virginia at one point passed a law that emancipated slaves had to leave the state within a year, but found that other states were reluctant to accept them. Missouri went as far as to ban free blacks from settling there. Even though it's about a somewhat later time period the book "What Hath God Wrought" by Daniel Walker Howe has some insights into the question and is the source of the above information.  

Two possibilities - it was an established policy and there was no feasible technological replacement to a manpower shortage.

I think it was a compromise when they were trying to prosecute a war of independence which required all the colonies to unify and stand as one. There was already some  recognition that there was inconsistency and potential hostility between northern states and southern slave states. But they ignored these differences in order to win a war. 

Because many of them supported slavery.

Founding fathers like Jefferson and Washington, who owned slaves, knew about the conflict but didn't know how to resolve it. Both figured it would resolve itself as tobacco became less grown by how it ate the soil. But, the invention of the cotton gin wrote history. Slave labor became more profitable than ever. 

There are plenty of books to read on this. My contribution is intentionally limited.

It was quite simple, in their eyes they were not human beings, and in the second it gave them thousands of people who had to work for nothing!

It was a different time people were educated but not as well as we , they thought different things about god about what he represented it was dreadfull in Europe during the reformation and whites were treated appallingly too disembowelled alive had them shown to them before being hung drawn and quartered , would you rate that fitting for believing some thing different in a different way , 

Why did almost every other country have slaves? Profits. Don't forget that we're all still animals.