5th pick is awesome, if someone from 2nd or 3rd drops down to 5th you just got yourself a deal.
And if no one drops down to the 5th spot, you are still in good position to bait any other team with to trade up.
st.Louis is very offensive minded, where as kansas city finally making it to the playoffs again last year, they want to try for the gusto again this year, so hence the high pick, to grab an impact player on more plays rather than a guy that just returns kick.
St.louis is probally confident in their run blking scheme and having a guy that can return the ball to the 40 or the 50 this way they wont have to march as far to get to the endzone.