Because some person made a decision (in the end it comes down to money somewhere for someone)
It could have been the engineer was bored and wanted to justify there paycheck to make small changes
It could have been decided that it would save money for the motherboard makers
It could have been the cpu cooler makers paid amd, as they benefit from having to sell a new cooler due to the small change.
It could have been necessary for some small reason that we shall never know
It could have been they just roll of a dice to make the decision
I do believe that motherboard manufactures have the final say in what support bracket is put on there motherboard because in the end they could leave it off all together just have the four holes and...