Why is "distro", rather than "distri", short for "distribution" in Linux world?

My guess is that distro might have been inspired by shortenings like repo[sitory], algo[rithm], memo[randum] &c.

Due to the Latin influence, "-o" is a much more natural-sounding ending for a singular noun in English than "-i". My best guess is that this subconsciously affected the coinage.

The OE has an extensive entry on the -o suffix ($) which I excerpt here:

The shortening of a word immediately after a medial o , and in particular where this occurs at the end of a prefix or combining form, first appears in the late 17th cent. and early 18th centuries, e.g. plenipo n., memo n., and hypo n.1 This probably established an association of the ending -o with casual or light-hearted use which it has retained ever since. Further examples are attested in the early 19th cent., e.g. (combining forms) Anglo n.1, mezzo n.1, typo n.; (other words) compo n.2, loco n.1 After 1851 this type of clipping becomes, and has remained, extremely common.