A few reasons: programming languages have universally settled on '.', numbers frequently end up in CSV files, and languages can't really support both standards, because of the usual function notation `f(a, 100.5, "c")`, a comma would be ambiguous here as it would be in a CSV. It's difficult and error-prone to interoperate between decimal period and decimal comma, and the former has only advantages over the latter.
Basically, it creates problems which no one needs. I don't consider this to be a case of cultural chauvanism, it's just that it so happens that the English-speaking countries chose the more useful representation when it became time to program computers. Separating arguments and expressions using a comma is much more natural than using a full stop would be, and this can't be parsed in tandem with a decimal comma.
You can't really count this as a win for the English way of doing things, which predates computer programming, the only context in which the difference actually matters. Before that, it was an arbitrary distinction, now it's not.