Lots of things can hold up the exchange of contracts.  The basic position is that the buyer will want to be satisfied as to what they are buying, so they will tend to ask questions and make enquiries of the seller.  These do take time to respond to.  The parties might be relying on third parties for something.  For example, a mortgage lender to confirm that they are prepared to proceed further, the results of a formal search about the property which has yet to be sent back to the buyer, a variation of a particular lease term in a leasehold transaction.  There is sadly no shortage of things that can happen and whilst the seller will generally be keen to proceed, especially if they have a linked transaction, the buyer will not exchange contracts until they are certain that they want to buy the property and are able to proceed.