I think the simplest explanation here is that pagers are small and light and don't have that much free space inside them, and it's hard to fit enough explosive into them to reliably kill people. The figures I saw was only a few grams of explosive could be fit in them. If you look at the photos and videos that have been coming out today you'll see what the injuries look like; they're not as catastrophic as getting shot with a bullet, or anything close to a real explosive with orders of magnitude more explosive in it like an artillery shell, rocket, aerial bomb, etc.

I would guess Israel would have preferred more lethal pagers, but the required amount of explosive simply didn't fit. So the resulting deaths are from the people who got really unlucky, whereas getting wounded was the modal result.