A bit of “fastest gun in the West,” a dash of inertia, and some catchy naming
Today, “cheater of Mystra” is nearly synonymous with “Initiate of Mystra,” the Player’s Guide to Faerûn feat for which it is named. That feat doesn’t really require optimizing—it’s just flat-out broken as-is. Hence “Cheater of Mystra” serves as a tongue-in-cheek nickname for the feat as much as it references a particular build.
But that wasn’t how the cheater of Mystra was originally presented: in a lot of ways, the focus of the original poster wasn’t even on Initiate of Mystra, but rather on Divine Metamagic and Persistent Spell. Which brings us to an important point: the cheater of Mystra is an early optimization build. Complete Divine was a new book at the time. Today, optimizers may think of Divine Metamagic (Persist) as a nearly-de-facto cleric class feature, but that simply wasn’t true in 2004. And while it isn’t likely that anyone missed the significance of Divine Metamagic for even a moment, the cheater of Mystra was a fairly early foray into optimizing it.
Likewise the dweormerkeeper—these days, it’s pretty well-known as a broken prestige class, and not particularly associated with the cheater of Mystra (as, truly, it’s broken no matter how it’s used), but in the cheater’s original thread, the dweormerkeeper drew the most comment and discussion. The prestige class was new, and people hadn’t really thought through all the things it could be used for. For many, cheater of Mystra was the first they’d heard of it.
And the cheater of Mystra directly inspired the twice-betrayer of Shar, which did similar things by abusing the Shadow Weave, and had even more convolutions to it. Note that the twice-betrayer of Shar is still an early optimized build, however.
Both builds aren’t really that stellar by modern theoretical optimization standards. There are loads of more effective options out there. On some level, both builds are famous more because of being there first, and having catchy names, than because of the actual result and how well that has withstood the test of time.
But being first matters; this is often known as the “fastest gun in the West” effect. Simply being the first to get into the details of Initiate of Mystra, Divine Metamagic, and dweormerkeeper gave the cheater of Mystra build a huge head-start on gaining notoriety. From there, simple inertia has kept it in the forefront of people’s minds for considering optimization efforts—though again, the catchy naming certainly helped.