imshow
doesn't support holding, like a plot would. So it is just printing over the previous image.
If you only have two images, you can use imshowpair
instead. Or assign a different color to each of your labels.
Or do something like:
mask=zeros(size(L));
figure;
for k=1:size(area)
if (area(k).FilledArea > 1500)
mask=mask+(L==(k));
end
end
imshow(mask>0);