Because DRK as an altcoin can easily mutate and adopt new features. Bitcoin at this point is pretty entrenched and I doubt we'll ever see widespread change to the software itself. It's locked into its current framework. Starting from this point, DarkWallet is already limited because it's tied to Bitcoin.

DarkWallet doesn't have a masternode system, which is probably one of the most versatile aspects of Darkcoin. DRK can also add more features to increase anonymity because it's its own individual entity. DarkWallet is directly tied to Bitcoin in the sense that the anonymity features will never actually be built into the Bitcoin software itself.

DarkWallet seeks to anonymize some transactions when people choose to use it. DarkCoin (in RC4) will anonymize the entire network, and do it in a way that makes it hard to do timing analysis to figure out who's sending what. The more people you have involved in the mixing the harder it becomes to trace.

Darkcoin's masternode system is directly incentivized by the network. People are being rewarded with DRK to maintain healthy masternodes. This means they have necessary selfish motivation to maintain them, as basic as that sounds.

I support DarkWallet and anonymity projects in general because it's necessary to have them. I just realize that having a decentralized blockchain-based solution built for anonymity is better than trying to piggy-back on Bitcoin.