- To long for a comment, so I added in extra details that might be interesting...
- I do not remember any explanation of why in the show.
- It was more than likely a tactical change.
- As you mentioned he also stopped wearing the duster.
- He still carried a bunch of hidden knives with him.
I don't remember ever hearing an explanation in universe, but I would expect it to be due to the tactics of the SG teams. As a soldier on Sateda he was likely (as we would describe) a typical line infantryman. He was capable of fighting as a unit, working towards objectives, etc. After the destruction of Sateda and becoming a runner those tactics were useless to him.
On his own, fighting only for survival, Ronon would need to be more stealthy and subtle in the engagements he found himself in. Getting up close is a good way to divide and conquer large groups and blades are silent. While Ronon did enjoy the straight up fight as much as the next bad@$$ (Teal'c for one) one on ten face to face would not end well, even for him.
"Teamwork is essential, even if only to give your enemy someone else to shoot at." - Unknown... probably some gamer...
SPOILER ALERT - Episode 3 season 4
For example, in the Sateda episode when Ronon becomes a runner again he draws the larger numbers of Wraith in to the warehouse where he has prepared for the engagement. Ronon is not stupid. He knows how to pick his fights and how to come out the other side capable to fight and kill more wraith tomorrow.
However, as part of a team Ronon would have to shift his tactics once again. In the case of the Stargate teams he would have to shift to the one fighting style his team knows. Fighting as a team. He could easily adapt to this thanks to his previous training and any adaptions he would need to make were hopefully minor.
Skip this if you dont want a mini lesson on military tactics...
Military combat on Earth has become much for of a ranged event. While most still have Close Quarters Combat (CQC) training (i.e. hand to hand) it is typically preferred to never exceed the distances of Close Quarters Battle (CQB). CQB is ranged combat at closer distances; I believe around four feet to twentyish (been a long time). This is clearing buildings, city fighting, etc. Having to fight up close and personal in CQC is much more gruesome, dangerous for both parties, and prone to "sticking with you" long after the battle has ended. The United States especially has adopted the idea of taking out "the enemy" before they even have a chance to do anything.
Anyways, Ronon would need to fight as a team at greater distances. The sword may have become an encumbrance, liability, or hindrance to him. This I think is also why he lost the duster as you mentioned it. The duster was likely a great asset when he found himself sleeping in the cold or whatever, but as a member of the Atlantis team he found it bulky and unnecessary.
Finally, I do not remember when he stopped carrying the sword, but I seem to remember him having an indefinite amount of knives hidden on his person?...