What is the reason for the recommendation for moving a baby to a cot once they roll? We have a large bassinet and our baby is 14 weeks and rolling- now unswaddled. Many thanks.

Research has shown us that once baby is rolling, they need the space & arms free to roll safely.
Many babies will start rolling (or attempting to roll) from 12 weeks/3 months of age.

Red Nose recommends modifying any swaddle from about 3 months is preparation for rolling.
Modify the wrap to meet the baby’s developmental changes, e.g. arms free once ‘startle’ reflex begins to disappear which is usually around 3 months;
As soon as a baby shows signs of beginning to roll, wrapping should be discontinued for sleep periods
https://rednose.org.au/article/wrapping-babies

It is also important that baby is in a cot for sleeps now, rather than a bassinet.
The extra space & arms free to roll safely is important.

Most babies will be rolling over by 4- 5 months - some earlier. This is very normal as babies grow & become stronger.
Many babies then prefer to sleep on their tummy.
This is not something that needs to be stopped. It is important to make his new environment as safe as possible.
Also, ensure that the cot mattress is firm & flat & there is nothing soft or loose in the cot,
Always continue to place baby on back to sleep.
Increasing tummy time when awake.

Red Nose has the following information now that your baby is starting to roll.

If you are using a bassinette, it is time to transfer baby into a cot as soon as they first show signs of being able to roll.
Give baby extra tummy time to play when awake and supervised as this helps baby to develop stronger neck and upper body muscles which in turn enables them to roll back over.
Make sure that baby is on a firm and well-fitting mattress that is flat (not tilted or elevated).
Make sure that baby’s face and head remains uncovered (do not use lambswool, doonas, pillows, cot bumpers or soft toys)

As babies grow and develop they become very active and learn to roll around the cot.
At this time, continue to put them on the back at the start of sleep time, but let them find their own position of comfort.
By this stage it is not necessary to wake during the night to turn baby over to the back position

Do not use any devices designed to keep baby in a particular sleep position.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over