Under a Resolution of the House of 2 March 1624, Members of Parliament cannot directly resign their seat. Death, disqualification and expulsion are the only means by which a Member's seat may be vacated during the lifetime of a Parliament. Therefore a Member wishing to resign has to go through the process of applying for a paid office of the Crown, which automatically disqualifies the Member from holding a seat in the House of Commons. There are two such offices: Crown Steward and Bailiff of the Chiltern Hundreds and of the Manor of Northstead.
Perhaps there were good reasons to prohibit members from resigning in 1624. But in the past 200 years, what has been the rationale to retain the system where MPs are not allowed to resign, in particular since they can de facto resign by applying to be Steward of the Chiltern Hundreds?