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Double battles are considerably more interesting to both watch and play for general audiences due to their faster pace and higher variety (primarily in how actions can potentially play out).
However single battles work better for a normal, solo playthrough where you are playing a game where you're on an adventure and you're finding wild Pokemon and such.
The difference between the two is also small enough on a surface level that there really isn't any reason to change this. Someone who knows how single battles work can intuit how double battles work, so the only difference is one or two small mechanics (spread moves) and the difference in strategy - and a difference in strategy is something that playrs would need to learn anyway when moving from single player to competitive.
