Why was the egalitarianism found in the Declaration of Independence considered hypocritical by some?

You are certainly right about the idea that slavery shows
the hypocrisy of the Declaration of Independence.  After all, the man who was the main
author of the Declaration, Thomas Jefferson, was a major slave owner who never freed his
slaves.  A person who could write that "all men are created equal" while enslaving human
beings is, by our standards, hypocritical.

I would also
encourage you to think about the status of women in American society at that time. 
While women were not enslaved, they were deprived of many of the rights that women take
for granted today.  This went beyond just lacking the right to vote.  Women were often
not allowed to own property or to enter into contracts or to do many other things that
adults are supposed to be able to do.

So, the men who wrote
and signed the Declaration were talking about equality for all, but they were (you can
argue) being hypocritical.  They were willing to demand equality for themselves while
denying it to others.