Like mad says, its all about money. You can go to Boeing and ask for 3% but it will coat you. Not many airports aroung the world where you operate airliners above the value of 2% so this was probably the cut off value. To do extra flight testing to get the a/c cert for 3% for a handful of airports where nobody goes anyway would have been expensive.
An interesting comparesen was the gravel kit for the 737-2, an afterthought for one operator initially I believe. In the process of flight testing and getting certification for this I was told about 10 engines were distroyed.
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