Hey!
The reason that's mentioned is because children who have not yet reached the age of 5 are a lot more vulnerable and basically a lot more dependent on factors around them. Once they have reached the age of 5, they are stronger and can withstand a lot more in relation to things like biological, sociocultural and environmental factors. This is the reason U5MR is used as a health status indicator because it's basically a reflection of the factors around the child, rather than anything to do with them alone because they are so dependent on their parents, their nation, the environment etc.
To put this into context, the reason U5MR is so much higher in low-income countries is because poverty means something like communicable diseases, for example, are more prevalent and they are the ones more likely to be affected as they are, by nature, more vulnerable.
Hope this helps!