Well, unsure for Java, but the standard Python implementation is called C-Python and is written in C language. So (if it was even possible) an OS written in Python would ultimately be built using C.

Moreover, when we think of programming languages, most are targetted at writing applications, not OS and programmers rely on the language implementation to interact with the system. The 2 major exception that I can think of are:

• the assembly languages: they allow to write directly machine instructions and can easily access the hardware - simply you must rewrite everything if you use a processor having a different instruction set ...
• the C language that can easily and with no external library access memory mapped hardware registers, and (provided you use it in real mode) can access the full system memory - C++ can also be used at same level. It was invented by Kernighan and Ritchie to build the first Unix OS, and first C versions really looked closer to a macro assembly language, than to a high level language.

On the other side, Java and Python are not intended to access memory at physical well known addresses and programmers use system libraries (often written in C) to interact with the OS.

Finally the C language has a poor reputation, because it often bites beginners or non cautious programmers because a program with no error or warning can still contains a lot serious mistakes and its power of accessing any memory address allow to easily use wrong ones. And writing a correct and feature rich program is usually much longer in C than in Python or Java.

What follow is just my opinion, only guided by ~40 years of programming in different languages: what makes the security of a program is not its language but the proficiency of the programmer and above all the respect of best practices. Among which:

• tests written against expected and pathological use cases (the program must behave sanely when given correct input, and just protect itself and the system when given garbage)
• extensive peer review - a fellow will point suspect code that you have written a bit too quickly because you were focusing on another point, much better than you would do
• use well established patterns (those will depend on the used language and frameworks) instead of re-inventing oval wheels
• only write code easy to read and understand, and add comments for the rationale and principes for low level optimizations

This indeed adds a noticeable overhead. But robust code comes at a price. And this is indeed a real reason to use Java or Python when C is not required, because you will use less lines of code, and they will be easier to read for peer reviewers. So a robust application program written is Python or Java will be cheaper than the same in C. But I really think that trying to use Java or Python for the low level OS parts is just non sense.