thomas jefferson was a slave owner. why, then, did he state in the
declaration of independence that " all men are created equal" ?

Well, it's true, isn't it  - except for the difference between G-A
Researchers and the rest of us humble humans. ;-)
Most of us make compromises in life.  
Although slavery is abhorant, good  - perhaps "good" -  slave owners
felt a responsibility for their chattels and weren't Simon Legrees,
providing for them and providing them employment in a time and place
in which it was assumed that this was acceptable, if not necessary for
them.
An interesting site that Pinkfreud posted to another question last
year pointed out that there were a significant number of English
immigrants to America who came as indentured servants, only a step up
from slavery.

"thomas jefferson was a slave owner. why, then, did he state in the
declaration of independence that " all men are created equal" ?"

Jefferson and all of the other Founders were a product of the times.
Lofty words may have been hard to implement in one generation. Slavery
was a complicated institution and Jefferson was a complicated man of
history. Both the Declaration of Independence and the Constitution
were a result of philosophical thought from the Enlightenment. The
manner in which language and ideas were used by the Founders was
entirely new.

ideal v/s reality

In the time of the Declaration of Independence the definition of man
was white, rich, land owner, and slave owner.  The people of that time
did not consider black slaves as men.  Slaves were considered as
property.