Quoting Windows Internals 7th Edt Part 1:

    There is an unfortunate side effect of large pages. Each page (whether huge, large, or small) must be mapped with a single protection that applies to the entire page. This is because hardware memory protection is on a per-page basis. If a large page contains, for example, both read-only code and read/write data, the page must be marked as read/write, meaning that the code will be writable. As a result, device drivers or other kernel-mode code could, either maliciously or due to a bug, modify what is supposed to be read-only operating system or driver code without causing a memory access violation.

There's probably no good reason to put code and data on the same page, it's just one extra TLB entry to use two pages instead so the data page can be marked non-executable.