The ease of cryptanalysis is caused by the feedback from the relationship between plaintext and key. A three-character guess reveals six more characters (three on each side), which then reveal further characters, creating a cascade effect. That allows incorrect guesses to be ruled out quickly.
the eaSe Of crYPTanalySIs is caUsed by The feedbaCK froM the RElAtIoNsHip betWeEn plAintext and kEY. a Three-cHaracter gUESs rEVEAlS six mOre ChAracteRs (tHree on eaCh siDe), whICH then reveal furtHER cHaraCTeRs, cReAting a caScaDe effect. that alLOws INCorREct guEsSeS to be RulED out qUiCkLy.