The original Hertzsprung-Russell diagrams constructed by Henry Russell and Eijnar Hertzsprung consisted of absolute magnitude on the y-axis and a spectral type or an indicator of spectral type on the x-axis. Below you can see an original HR diagram produced by Russell in 1913.
When the diagrams were constructed, it was not at all clear what the sequence of spectral types or spectral type indicators actually meant. It turned out of course that the sequence (in modern day parlance O,B,A,F,G,K,M) actually corresponds to decreasing temperature.
Astronomers have simply stuck with this convention to the present day, there is no particular reason for that. Most HR diagrams are now plotted with temperature (decreasing) along the x-axis, although that is not what the original HR diagram was.
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