I guess because the demand for 5 Pfennig coins in 1909 wasn't high enough to mint them in Karlsruhe. This also happens with other coin types and numerous years (but not with all skipped-date coins!). For example, this phenomenon has also occured in 1974, with the 1 gulden from the Netherlands. https://en.numista.com/catalogue/pieces738.html In 1974, the demand for 1 Gulden's was simply too low so the Royal Dutch Mint skipped that year minting them.
Hope this helps you !
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-Plato