I think its a combination of the lack of sequencing and the nature of exponential spread. I also think that we probably were the source of it, or at least one of the first countries to get it.
1 .. 2... 4... 8...16...32....64....128...256....512....1024....
A new variant would potter along at pretty low levels when it started out somewhere new, until suddenly BOOM! You REALLY know its there. Particularly if the other strains are in decline around it.
I think the new UK strain is still relatively recent, and is probably just hitting the BOOM! stage here (despite having been around for a couple of months) and is probably still in the pottering stage elsewhere.
The South Africa strain is going BOOM! over there, but is still pottering here.
Catching a new strain in the early stages is like winning (ha!) the lottery. The UK is doing a tonne more sequencing, so yes, we're discovering it more quickly. But I think other countries will notice it now even without sequencing, because firstly, the deletion in it makes it possible to spot through testing without fully sequencing (though you wouldn't be able to tell between UK and SA, from what I understand) but also, because it explains unexplained spikes and has a really recognisable growth pattern compared to the old strain. It's easier to find something once you're actively looking for it.
I expect in the next couple of weeks, we'll hear lots of "New strain found in France, new strain found in Texas" stories.