Lackluster compared to what? Because in its field, watches in general, it's the #2 best selling watch. And in its niche, smartwatches, it's outselling the competition (Samsung, etc) by an order of magnitude or so.
It also became a multi-billion dollar business just as it came out -- and it's only in it's 2nd release (it took more time for the iPod to sell the same amount of units). And of course, compared to the profits coming in, there's probably not even a comparison, as Apple is unique in having high profit margins whereas many else are selling commodity hardware on thinner margins.
The main issue is people having unrealistic expectatations of what the sales of the Apple Watch should be. Obviously everybody has a smartphone (whether Apple or not), but very few will have a watch (whether Apple Watch or anything else). Heck, most people don't even wear an ordinary watch anymore.
So, given that, and given that Apple never said "the Apple Watch will surpass iPhone / iPad etc in sales", they just released it as another product they do, it's incredibly successful. In fact it just sold more units this quarter than it did last year, so it keeps improving too.