An important factor is simply time. Handwriting styles change over time, and the cursive styles used in the past are especially hard to read for modern readers,

An important factor is simply time. Handwriting styles change over time, and the cursive styles used in the past are especially hard to read for modern readers, since as we write less and less by hand, cursive writing (which is optimised for speed of writing) is rapidly falling out of use. And since we're not used to them, they're hard to read.