You could do topic branches in SVN, it would just be painful. It's sort of like saying "Why use C++? You can do object oriented programming in C with structs and function pointers."
This is largely due to the fact that I have a strong aesthetic preference for languages where you can hold most of the language [1] in your head. Such languages are called compact [2].
[1] Compactness of language and standard library are different. Having a non-compact standard library isn't necessarily a bad thing, especially if it means I don't have to complicate my life with third-party libraries.
[2] http://www.catb.org/esr/writings/taoup/html/ch04s02.html