Why are 555 timers so prevalent?
I'm not a 555 fan and only used them a couple of times over several decades but they do have their uses.


  Why is the 555 so prevalent, or ubiquitous; what about it so useful?



It was probably the first general purpose timer to market. (Timing is everything!) It was featured in all the hobby electronics magazines of the 1970s, etc., so it became very well known.
"555" was easy to remember. (Don't laugh!)
It has a wide working voltage range - 4.5 to 16 V. This beat any of the logic families.
It was flexible. Oscillators, monostables, pulse generation, sequential timing, time delay generation, pulse width modulation, pulse position modulation, linear ramp generator, etc., could be formed with only a few components.
Due to the 1/3 and 2/3 Vcc switching points the timing was pretty much independent of voltage. This is useful for battery powered circuits in particular.
(Only?) 50 ppm temperature drift.
200 mA output source or sink capability. Again, this beats any of the standard logic families and allowed direct drive of LEDs buzzers, speakers, motors, etc.




Figure 1. Internals of the 555. Note that all transistors are BJTs. 


It was tough. The device is built using regular transistors (see Figure 1) which are fairly tolerant to electro-static discharge (but see 11.2 on datasheet below).
Price is very low.
For all of the above reasons it made a great chip for "my first project" tasks. A capacitor, a few resistors and an LED were all that was required to get someone started on the road to becoming a major EE.SE contributor.


Ref: TI 555.