By no means all non-US phones are dual SIM - in fact only a minority in most countries, though it's very popular in some parts of Asia.
Dual-SIM phones have advantages for people who need to keep 2 phone lines on different networks. This is useful for people who share their time between two countries, and has advantages for people who travel more widely (cheaper calls using the local SIM, while still being contactable on your primary number, albeit paying roaming charges for that). Some dual SIM phones these can be used either with 2 SIMs or 1 SIM + 1 SD card, in which case you need to decide what your priorities are.
If you don't need dual SIM then there's no major drawback in having this, but also no actual advantage.
As for why US models are rarely dual SIM, an obvious answer would be that the US handset market is still heavily controlled by the carriers, and they do not want to sell phones that allow you to include a second SIM in them. After all, when you are at home you could be using that with a rival carrier, and if you are travelling you are making calls on a local network rather than paying their international roaming rates, neither of which they want you to do.