What can be their reasons?
Whatever that is/are (P.S - nicely explained in the other answer by mhoran_psprep), you have a very solid reason to decline this and continue with the job search - this is not normal, if not a scam altogether.
They can have a thousand reasons / justifications, but you don't have to listen / comply to any of them. An employment is a two way street, if one party is not going to be happy by the agreement, the relationship is not going to last long anyways.
If they cared about their employees, they would ensure that the employee is satisfied with the contract before signing and joining, not to be tangled in the back and forth process after joining and risk ending up unsatisfied. No half-moderate organization would want that, as recruitment demands a ton or resource, time and effort, so to have someone onboarded just to risk letting them go in a short while because of below-par contract agreement terms is counter-productive, unless they gain something substantial out of that short employment (like free labor).
TL;DR- Not worth your time, move on.