It likely had to do with the events in the first year of TNG, and his commanding officer(s). First, a captain has final say on who serves on their ship. The Enterprise, as the flag ship has the best of the best wanting to serve. Picard as shown, is a very open minded person. He allows cultural and religious exceptions to dress code, runs an easier type of crew shift, prefers ambassador/diplomatic and scientific approaches over the traditional military approaches. He would have jumped to have Worf aboard.
Even then, Worf started as a command division junior bridge officer, replacing the main bridge officers when their duties called them away from the bridge. Of course, as a Large, Strong fellow, he was still pegged for the occasional show of strength. It was a year later, when Tasha Year was killed off, that Worf asked, or Picard or Riker asked him to become acting security chief. As the executive officers, again, they have the final say on ship operations. This would be over those already in operations division acting as security officers. This served Picard well for diplomatic relations with the various Klingons encounters in the series. It would have been a calculated move. It took a year as acting security chief before Worf switched divisions and became the official role, again, under Picard.
Picard would not be the type of person to let the federation meddle in his personal and professional decisions in who he allows to serve in what capacity he chooses on his ship. Such as Data and Wesley.
We see that not all of star fleet is as open minded. The Admiral who took over for Picard overrode Riker objections to how to run the ship crew shifts, the dress exceptions, etc. Likely would not place Worf as chief of security.
But at this point, there are some diplomatic tensions between the federation and the empire. Had they learned that the federation deprived Worf the position, it would not have gone over well.