Part of the reason it's historically sold so well is that its particular combination of features is difficult to get economically in other ways. If you want, say, a blinking LED without a microcontroller, it can't really be done without a 555 except at severe penalty in cost, complexity, or both. Any analog solution involving an op-amp, for example, has to deal with the fact that an op-amp is usually more expensive than a 555!

(Example: TI's cheapest 555 is the NE555 in TSSOP-8 package at $0.05, and their cheapest op-amp is the LM321LV in SOT23-5 package at $0.04, actually slightly cheaper than I remember last time I went looking for the cheapest op-amp money could buy. But the op-amp will require more supporting components, increasing total cost.)

That's not true for voltage regulators: there are many other parts that do what a 7805 does and many ways to build that function, some of which can sometimes end up cheaper.

Of course, the previous caveat of "without a microcontroller" is quickly becoming a concern of the past, and, accordingly, the 555 isn't designed in nearly as often as it was even in 2003.