The most obvious reason is portability. Even though it's been nearly twenty years since the C99 standard, some compilers still haven't implemented VLAs. To make

The most obvious reason is portability. Even though it's been nearly twenty years since the C99 standard, some compilers still haven't implemented VLAs. To make things worse, the next revision of the C standard (C11) made VLAs optional, so these compilers now have an excuse to never implement it. But even discounting that, there are still some issues with VLAs. For instance, the Linux kernel decided to remove all uses of VLAs they had, because they generate worse code than a partially-used fixed-size buffer, and also make it easier to overflow the kernel stack. VLAs, like their ancestor alloca(), seem convenient but often are more trouble than they're worth.