Probably because the unit bearings suck, and if you break a front axle, you can unlock the hubs and drive, instead of taking it apart.
You are very unfamiliar with breakage and hub conversions, aren't you?
Three things break in the front axle with the small conversion.
Hubs, and although you could drive that way, do you really want all the mud/dust/dirt/whatever getting into the outer bearing? Not really, and it's also very rare that if you are in a place where a hub breaks, you don't still need 4 wheel drive. So, you're fixing it to get home.
U-joints- As above, when and where would you break one that you don't still need 4 wheel drive? Yep, you're fixing it or getting towed to where you can drive in 2 wheel drive.
And finally, the inner axle breaks and yep, you guessed it, tow until you can use 2WD or fix it.
I am a big fan of hub conversions, I own or have owned many many of the Warn versions for years and still currently have at least 2 complete ones floating around. We've also run the crap out of them in JV where they are very nice to have.
Nowadays though, the only good reason to own one is so you can finish out the trail in 4wd with a bent front driveshaft and then unlock the front to get back to camp or home.