As mentioned system reliability and ability to hot or cold swap units on site could be a major driver for paralleling power converters. If three phase power is the source of power directly rectifying the three phase line to get and input DC voltage, can in my experience, be done while getting very close to 0.9 power factor with little filtering. The rectified DC can then be used by one or more power converters and would have much less filtering than if a power converter
was placed on each each of the three phases with their outputs in parallel.
While multiple supplies are touted to be more reliable it is actually the availability of output power that can be improved by paralleling power converters and that can have a marketing advantage or be a requirement. The actually failure rate would increase as you have more parts to fail.
As far as cost goes I believe at low production volumes multiple parallel supplies can give you flexibility to use in potentially more applications. That is a 50W converter can be used in 50W applications and two used in 100W applications etc.
If the product volume is low then using lower cost parts in larger volume may not cost as much more as many people assume; not including development cost which might tip the balance making lower power parallel supplies less expensive. And as said multiple supplies tends to spread the power distribution, which may or may not be an issue.
A lot to consider but I think the market is the key to your answer.