“After a K1, why should someone apply for a 10 year green card instead of naturalization?”
Before you can apply for a naturalization, you have to be a legal resident first for 3 years if you obtained your green card through marriage. After entering on your K1, then marrying your fiance, you’ll adjust status and get your conditional 2 year green card. This is when the 3 year clock starts for your citizenship eligibility. By doing a little basic math, you’ll realize there is an extra pesky year in there that you’ll need a green card renewal. But at this point, the only green card available to you is the 10 year card you’ll get after removing conditions. So while your green card will be valid for 10 years, you can still apply for citizenship after that 10 year card is only 1 year in.