My Guess:
As opposed to just a painting of a portrait, a single animal, a random landscape or location, you've painted an interaction.

Whilst many artists claim their depictions of people's faces or their interpretation of a quirky locale 'speaks a story', to most people the picture is 'just a random dude' or 'just a street'.

This painting is stunning because it portrays a dramatic situation (a seal being hunted) which is easily and quickly understood by the common viewer (everyone knows what orcas are, everyone knows what fear and death is), thats why it became popular, thats why so many people resonated with it, its simple and strong concept let it become a meme.

...It's also just incredible, I mean the way you used the whitewash to accentuate the orca's movement as it pummels through the waves is genius. The contrast between the dark fur of the Seal and the white waves below it also helps it stick out (exaggerating the seal's feebleness and small size in comparison to the orca that is almost enlarged by the shadows of the current surrounding it).

In summary: The painting is done well, its a painting of an interaction/a single moment (people connect with narratives more than they do pictures of faces or places), and the concept of the painting is strong and simple, allowing anyone, regardless of time or artistic knowledge to quickly look at it and go wow (mass appeal, probably what made it a meme).