Voyager was operating under unusual circumstances with a relatively small crew, but it did have an ace pilot

The crew of the USS Voyager, Maquis included, was about 120. Kirk's Enterprises had crew complements of roughly 3 to 4 times this many (depending on which Enterprise and which decade of operation), and Picard's Enterprise had almost 10 times this number of crew. Naturally, those ships would have more trained pilots than Voyager does.

Let's discuss this in a little more depth, focusing on the Enterprise-D as a ship that was nearly contemporaneous to Voyager. Being the flagship, the Enterprise-D was not on a long-term deep-space mission. It was freqently visiting various Starfleet starbases, and during TNG we see it visit Earth a number of times.

New junior crew members, often fresh out of the academy and requiring experience, were acquired by the Enterpise-D at various points. Riker and Troi were in charge of the bridge duty roster, and the "night shift" was frequently reserved for officers, including helmsmen, with less experience. Sometimes, however, an emergency strikes while a junior officer is at the helm, and sometimes another officer is seen to take over. All in all, there are plenty of reasons why we see frequent changes at the helm on the Enterprise.

Now let's turn to Voyager:

• Voyager is stranded in the Delta Quadrant. The crew it has is the only crew it is going to have for the foreseeable future.
• Of the only 120 or so surviving crew members, only a handful would be competent enough to fly a starship through unpredictable circumstances. (In normal circumstances, the ship can fly itself if necessary, and the computer can also implement pre-programmed evasive maneuvers if necessary, but a competent helmsman is required for most real battle scenarios. That's why starships still have helmsmen.)
• Voyager's helmsman with the most experience was Lt. Stadi; however, she was killed when Voyager was flung from the Badlands to the Delta Quadrant in the first episode.
• Without being ordered to do so, Paris grabbed the helm and steered Voyager to safety.

Given Paris' expertise, and given the shortage of personnel, it was prudent for Janeway to make Paris into the Chief Conn Officer.

Upshot

I know that you asked for an in-universe reason given by the producers, but my assessment is that the in-universe reasons for the conn officer position are clear enough from Voyager's very particular situation.