I have owned and towed caravans on live beam axles for over 20 years, the last of those being a 2008 Jayco. I never had issue with the beam axles. Our current van came with the simplicity independent and I have found it tows considerably better, simple things like not getting a wobble when one side of the road is damaged, not bouncing when on corrogations.
Again, there is nothing wrong with beam axles, however, independent is nicer.
Why is it not standard fitment? Jayco are a budget builder and this suspension is an extra cost of $2000. I am certain that people would look at other brands with an increase in cost. Many of the high end builders do include independent as standard, however do you spend $40k on a jayco or $100k on the others.