Because traditionally Cancer is represented by the crab. The crab is a non vocal animal ( it has no voice). Any sign of distress it retreats into its shell.
However though outwardly harmless and non threatening on the outside they may be, crabs are fiercely protective and territorial creatures. If they feel threatened, they use the pincers and cling on - the pincers are powerful and can cause pain.
Cancers in general ( much depends on their rising sign and Mercury) are shy and reserved, as they take time to trust people and to open up. They have, a fun and caring side, but to see that they must trust you and feel comfortable in your presence. They are not as outgoing and gregarious as say a Leo or Gemini may be, but they usually make good friends and even better family members.
No two Cancers are similar though ( aside from being devoted to their parents) I have two Cancer acquaintances. One’s a friend, another is a prof I am close to. My Cancer friend is friendly, outgoing and charming, working on her startup and looking after her family.
My prof on other hand would qualify more to the classical ‘’Cancerian’’ traits - she is very emotional, prone to moodiness, freely expresses it before her friends, is reserved and is very imaginative/creative . But both are delightful women in their own way.