Because (and what should be immediately and deeply and naturally obvious to any Catholic), it means that God is granting us a great dignity - far beyond any other animal that He created.  

God is asking us - commanding us in fact - to share in His own creative & problem solving attributes. 

He gives us "capacities" to develop and use...our will, our intellect, our conscience (which is a function of the intellect), our vitues, etc.  to serve selflessly His will, & to serve the great good of all creation.  

This means a lot of "problem solving" for others...serving them, using well the resources that God put at our disposal to serve Him; not to keep buried, as the parable of the talents describe. 

To properly "subdue the earth" & "to fill it" demands that we must first "subdue" our inclinations to pride, to vanity, to endless comfort seeking, and to all manner of sin.

So it's a very "stretchy" command that God gives us, one that will allow us to become ever more like His son Jesus!