becaaause.. it's intermediate scales are un-prefixable, because it names adjacent intermediate orders of magnitude with only subtle mutations of the previous name, because it's inconsistent with itself (<x 10^9), and finally because in 2019 it's extremely ambiguous, in all languages SI prefixes have been around since 1960 all of science and engineering uses these. In other words the same reason English abandoned the long scale.

Then again it was probably far easier for Britain to change in 1974 than it is for the remaining long scale countries to change today since the discrepancy will have been built into all software and systems dealing with currency and other things between countries since then... So i guess you are probably stuck with it and all it's disadvantages whether you are aware of them or not.