Why does time stop at the 'Event Horizon' of a Black Hole?

It does not “Stop”, though the passing of time does slow for an object that is closer to the greater source of gravity in relation to one that is farther away. The best way to think of it would be rather like two turning sprockets – if you imagine them as two different sources of gravity and the smaller one is the denser (and therefore has greater gravity) of the two and thus has more rotations though the two sprockets are spinning at the same rate of speed. Do keep in mind that, if you were actually observing sprockets, the effect is reversed; time is slowed in relation to it’s proximity to a gravitational source, not – as in observing two sprockets - rotating faster, but you can some-what get an idea of the appearance of time’s passing in relation to multiple objects and their respective proximity to a source of gravity.

This is no great elucidation of the phenomena, but it will have to do for a two-minute response to you question.

It doesn't necessarily stop, it just ceases to be an important factor in the continuum; human-defined time is based on what we see. We have broken time down into seconds, minutes, hours etc. i.e. measurements. The reason time will cease to "stop" is because the most important thing is taken out of the equation at the event horizon: light. Light is pulled into the black hole and, as you know, the Event Horizon is the point of a black hole where nothing can escape once something has reached it. Since light is pulled in and becomes null and void, time will be distorted and will not follow the measurements we humans have defined for it.

It doesn't.

An outside observer observes clocks approaching the event horizon to become asymptotically slower, but he's far from the horizon observing this happen from a great distance over a tremendous amount of curvature. In other words, his observations don't count for a whole lot.

A graduate student falling into the black hole notices nothing at all amiss about her clock as she crosses the event horizon.

Time is relative, it only appears to stop for observers on the outside of the the event horizon.

You may have read that as you approach the event horizon of a black hole, time will stop for you.

Indeed, for any observers, it will appear that you go slower and slower and never quite reach it (ignoring the problems that you would be crushed to death by the gravity!)

However, if your watch were still to work, you would look down and perceive time to be passing as normal.

It is due to the immense gravity that the light you emit takes longer and longer to reach the external observer.