Technically, serial monogamy is what most monogamous people do: being monogamous to one person at a time, but not one person their whole life. If you look at biology, they call monogamous the species that mate for life (only one parter) and serial monogamous those that pair up, but whose pairs can change along their lives.
But with what is called "serial monogamy" with other species being pretty much the norm for humans, and people calling it simply "monogamy", the term serial monogamy has been kind of re-purposed to meaning someone who has a quicker succession of partners. A way to have several partners without having them at the same time, if you will.
Depending on who you ask, "quick succession" could mean every few months, or it could mean every few years. There is really no rule.
The fact is that these people are serial monogamists, since they have several monogamous relationships over their lifetime. But it has been used a lot for people who are sometimes considered to refuse commitment, or not want to settle down, or whatever.
In truth, it doesn't really matter why people have the partners they have, provided they are honest about it. As a result, I'm very uncomfortable with the idea of "serial monogamist" ever being used as an insult.