TrueCrypt is not really Open Source (ie not accepted by OSI) nor accepted by FSF as Free Software. But it's gratis and its source code is available, and the authors claim that is both Free and Open Source (that is wrong). Anyway most of people do not care about the strange license and just use it.
On the other hand, SD4L is kernel-dependent (depends on the version of linux kernel) and the GUI is based on Qt3 as far as I remember (and If your kernel is not suppoted by binaries, you have to install qt3 development files and compile SD4L yourself... that is much difficult). And totally it seems to be a dead project. This is probably why no much people are using it rather than TrueCrypt.