I believe an opportunity was missed some time ago in the past to define one inch to 25.6mm, which would make for much nicer conversions IMO, especially with the convention to use fractions of powers of two in the inch world (e.g. 15/16" would be 24mm exactly vs. 23.8125mm).
Usually the absolute value of units is preserved as much as possible when a unit is redefined.
In the 1800s when Britain and the USA stopped keeping their units in alignment, they both defined conversion factors in terms of inches per metre (the inverse of the modern definition). However the conversion factors were slightly different. In the 1920s and 1930s, Carl Edvard Johansson used 25.4mm as a compromise between the two definitions when making his guage blocks, and this became a de facto standard that was later blessed as the international standard inch.
25.4 mm, ever since 1959 when we redefined it.