Scientific detection is 'too easy'.

 For all its associations with sportsmanship, cricket is governed by complicated laws, and Milne reckoned that if the detective novel was a game, readers and writers need to know the rules. When The Red House Mystery was reprinted, he set out half a dozen key points:

1. The story should be written in good English.
2. Love interest is undesirable.
3. Both detective and villain should be amateurs.
4. Scientific detection is 'too easy'.
5. The reader should know as much as the detective.
6. There should be a Watson: it is better for the detective 'to watsonize' than soliliquize.

M. Edwards, The Golden age of murder (2015), 113