We have a 6 week old who loves sleeping on his side, and doesn't sleep as well on his back. We are aware of recommendations to sleep babies on their backs. What is the reason for not sleeping on the side? Is this because they might roll onto their front and potentially suffocate? Or is it harder for them to maintain a patent airway when lying on their side? Some other reason? Hard to find the rationale for this recommendation.

Red Nose has the following information about side sleeping
Placing babies to sleep on the side doubles the risk of SUDI (SIDS and fatal sleeping accidents) and is not recommended as a safe alternative to sleeping a baby on the back. The side position is unstable: a baby is likely to roll into the high risk tummy position, placing the baby at nine times the risk of SUDI.
https://rednose.org.au/article/can-baby-be-slept-on-the-side
https://rednose.org.au/news/why-should-you-sleep-your-baby-on-their-back
There are some concerns about baby’s airway when placed in side position, due to the position of airway (trachea) & oesophagus.
Placing baby on back for sleep is safest for baby.