Why is it that spoken Italian seems easier to understand than spoken Spanish?

There’s a slight factor, which Chris Lo has already pointed out in comments, but it’s only slight.

Spanish does not have length contrast in vowels or consonants. As a result it is syllable-timed, and it is spoken quite fast.

Italian has audible vowel length differences (stresses vs unstressed), and also long and short consonants. That makes it spoken a bit slower, and there’s more phonetic variety, which (for me) makes it a bit easier to pick out words.