And probably because signed integers are a default choice in certain languages and/or maybe on certain architectures.
Java, for example, famously doesn't even have an unsigned 32-bit integer primitive type. (But it has library functions you can use to treat signed integers as unsigned.) Ultimately not a good design choice, but the fact that it actually wasn't that limiting and relatively few people care or notice tells you that many people have a mindset where they use signed integers unless there's a great reason to do something different.
Aside from just mindset and inertia, if your language doesn't support it well, it can be error-prone to use unsigned integers. In C, you can freely assign from an int to an unsigned int variable, with no warnings. And you can do a printf() with "%d" instead of "%u" by mistake. And I'm fairly sure that converting an out of range unsigned int to an int results in random-ish (implementation-defined) behavior, so if you accidentally declare a function parameter as int instead of unsigned int, thereby accidentally doing an unsigned to signed and back to unsigned conversion, you could corrupt certain values without any compiler warning.
