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The nmos and pmos transistors have different placements for their source and drain anchors (for nmos, drain is on top and source on bottom; for pmos, source is on top and drain is on bottom). Why is this?

\begin{circuitikz}
    \draw (0,0) node[nmos](n){}
    (n.gate) node[anchor=east]{G}
    (n.drain) node[anchor=south]{D}
    (n.source) node[anchor=north]{S}
    %
    (3,0) node[pmos](p){}
    (p.gate) node[anchor=east]{G}
    (p.drain) node[anchor=north]{D}
    (p.source) node[anchor=south]{S};
\end{circuitikz}

enter image description here

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    From Device Physics the function of Source and Drain are interchangeable. However, protection diodes may be applied asymmetrically, and are not shown, which make S and D less interchangeable. // From an electric perspective the hidden message is where to put plus and minus poles from the battery ... which depends on the MOS-transistor type used. From this perspective it's a conclusive definition.
    – MS-SPO
    Jan 21 at 6:52

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Because this reflects the most common usage, where the source of the channel-p is connected to the positive (up) rail, then going down you have a channel-n which its source connected to the ground rail.

The same happens with NPN and PNP transistors.

It's basically a design choice made a lot of years ago, and in some way arbitrary, but it's so old that's basically as if it were set in stone.

Addendum there is a notice in the Changelog, for version 0.6 (2016-06-06): enter image description here

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