why interrogative form of construction like "Should you work hard" is used instead of "If u worked hard"?
Your two examples are not equivalent, because Should you work hard? is a complete interrogatory statement, and If you worked hard [...] is incomplete since it lacks the requisite conditional result. This leads us to the answer to your question.
The form Should you work hard? is used in order to avoid the necessity of stating the conditional purpose. When if is used to introduce a question, the conditional purpose must be included in order to employ standard English usage. For example, If you worked hard, then would your family have food?
The word should may be used to introduce a question because, as you have said, it can "express condition" (Merriam-Webster, definition 1). When you use it to form a question, as in Should you work hard? the usage implies a purpose Should you work hard [if your family is to have food]? (This also uses Merriam-Webster's should definition 2: "to express obligation".)
The word if can introduce a question, when it explicitly states the purpose that a "should" interrogatory merely implies. To repeat the previous example: If you worked hard, then would your family have food? This is because if can mean "on [the] condition that" (Merriam-Webster, definition 1d), and thus it can also be a conditional, just as should. Usually, the structure is an if...then (interrogatory) statement