Probably because the shopping experience in Target is more pleasant. It's quieter, the furnishings and the like are a little more quality than "one step beyond cardboard", and the checkout lines are efficient, as opposed to wal-mart's third-world queues.
A lot of this depends on individual stores and regions.
My store for example, well it's a Wal Mart make no mistake so don't expect a miracle, but the rank and file worker bees are college students. Most of us are grossly underemployed. I know I am worth more than I get paid here, but working here is the opportunity cost of getting that MBA I want so much. A lot of my coworkers graduate college a couple years ago and just can't find a job in their field, and some have a useless degree like Psychology. A lot of them are going to the community college or are trying to find a way to go to a technical school. A couple of them are people like our own RileyMC who actually have pretty extensive white collar careers but fell out of them for whatever reason and realize that $8 an hour > $0 an hour. Our checkouts average something close to 450 items per hour (Lowe's next door is like three hundred or less). There are stores that do a little bit better, and there are stores that do a lot worse.
That's not saying our store is great. I have no idea what the difference is between most of our wares, I can't tell you how to take care of the live fish we sell for example, and I will tell customers straight up when I do not know something, or if something we sell is garbage. However, I also sincerely believe people come here because they want a cheap piece of Chinese crap jigsaw or whatever they're only going to use once and then never touch again and know what they are getting into. Most of my coworkers can manage a complete sentence and our store generally performs well, our customers seem to rate us highly too.
My aunt, who's assistant manager of a Wal Mart in another state and a career Wal Mart employee, works at a store where the typical employee probably barely has a GED and the cart pushers are all convicted felons. That store is a mess and it's everything she can do to keep it standing. However, since it's generally easier to improve a really bad store than to ramp up a generally good one (firing the right people helps), it's a good opportunity for her to get some raises.
Another problem is that if you have a store in a place no salaried manager wants to live for whatever reason (I live in a fairly pleasant Texas municipality of about 100,000), your store manager changes every 3 years or less. I've worked at one store where that was the case because everyone wanted to move on to a bigger store with bigger profits (the manager's bonuses and pay were tied to profits and larger stores in bigger cities typically do better). I don't think Wal Mart pays its managers that way, though, but a lot of people want to work and live somewhere else regardless. When that changing of the guard happens, the people who get lost in the shuffle tend to be the most experienced as the incompetent new manager promotes his own toadies up to replace them.
Wal Mart is in no way unique though, all big box retailers are like this.
