Good question, Graham. This is simply because WinPcap has taken the
System32\SysWow64 dirs and NPcap wants to coexist with WinPcap. As NPcap has the same file names (wpcap.dll and packet.dll) for compatibility, we just can't put the the-same-name files in the same folder with WinPcap. Though I personally prefer the way to "Make NPcap and WinPcap mutually exclusive" (this needs user softwares like Wireshark and Nmap nothing to change), coexisting way has also its benefits, and finally we chose the latter.
Cheers,
Yang