Why is the word "triple" spelt with 1 p although tri is an open syllable?

nipple has a double p. tripod and triangle are pronounced tr/I/

Plenty of English words have a non-doubled consonant letter after a short vowel. The only time when you can expect consonant doubling to systematically apply after a short vowel is before certain suffixes (such as -ing, -ed, -er, -est, -ist).

But in a non-suffixed word like triple, there isn't an easy way to predict the spelling from the pronunciation or vice versa. The spelling "tripple" was used in the past; it happened to not survive up to the present in standard written English. When English spelling became more fixed, a lot of relatively unpredictable quirks like this became frozen (the spellings used in Samuel Johnson's dictionary were often adhered to by later writers). The fact that the French and Latin etymological sources, triple and triplex, have only a single P was possibly a factor.