> Lines of Quoted-Printable encoded data must not be longer than 76 characters. To satisfy this requirement without altering the encoded text, soft line breaks may be added as desired. A soft line break consists of an =

IIUC in Base64 you can throw whichever white space anywhere and it should be ignored. And in URL ("percent") encoding there is no insignificant white space possible (?) and encoding of white space depends on implementation (dreaded space `%20` vs ` ` vs `+` in application/x-www-form-urlencoded [2]).

[1] https://en.wikipedia.org/wiki/Quoted-printable [2] https://en.wikipedia.org/wiki/Percent-encoding