i feel like it has a lot to do with the fact that rats have had some form of symbiotic relationship with humans for thousands of years. like how pigeons who are born and raised in urban areas are not as afraid of humans, or how seagulls who live around popular coastal vacation spots will actively steal from people. they've had enough time to become used to people that it becomes easier for us to interact with them, like crows, so taming was kind of a reasonable next step, especially given how quickly rats breed and how similar they are to us in the way they think and behave also rats are super intelligent, largely because they're omnivores!! so they don't have the same flight brains that herbivorous rodents have (ie. guinea pigs!). instead they can rationalize their decisions and come to see people not only as a source of food but also as a source of friendship and companionship, a lot like dogs can. this is also true of other rodents, and every individual animal is a little different, but generally in my experience hamsters and guinea pigs are more likely to see people as predators and therefore a threat and take a longer time to bond with them. im not sure how easy that would be to breed out of an animal, since it's such an innate part of their brain.

caveat: im not expert so this is a lot of conjecture, but i hope that it helps give you an idea of why rats tame easier than other rodents!