This isn’t really a “once in a lifetime” event; volcanoes erupt all the time and this one wasn’t particularly huge. In fact, there are 46 currently erupting with an average of 20 active on any given day [1].
A few times a year at least; this one was just more visivle than most because it happened just under the surface in an area of more or less open ocean, so there was nothing to deflect the shockwaves. It also started erupting in December so there were already satellites paying attention to it.
> Early data from Tonga's violent volcanic eruption suggests it is the biggest blast since Mount Pinatubo in the Philippines 30 years ago, volcanologist Shane Cronin says.
> It also started erupting in December so there were already satellites paying attention to it.
All images I've seen are from satellites and/or constellations that watch the whole Earth or a fixed part of the Earth, none have been tasked to specifically watch this area because of the eruption.
[1] https://volcano.si.edu/gvp_currenteruptions.cfm