Why do crypto exchanges impose withdrawal limits for BTC and other coins
Limits could be in place for a number of reasons, such as:


Regulations imposed by the government of the country the exchange is operating within (eg unverified customers can only withdraw a certain amount)
The majority of the exchange's coin reserves are held in an offline, cold storage wallet. So a very large withdrawal may require more coins than the exchange has available in an automated hot wallet (this is a security consideration)


Every exchange will have its own considerations in this regard, but these examples should give you some basic understanding of why limits would exist.