sorry I think my reply does not explain properly why you are likely to find such letters in a CRO, i've been mulling it over whilst a cooked breakfast and decided I need to elaborate.
The first point is that anyone able to actually write a letter in the 1700s would have had to have been educated and therefore of some social standing.
Secondly, this probably meant that they were from a gentry family at least, and gentry families traditionally kept all correspondence over time in their estate office.
During the last 50 years, the facilities offered by the County Record Offices have meant that family and estate papers could be deposited on loan for safe keeping, or donated and the donation anotated in the CRO catalogue.
This means that original letters are accessible by everyone (with a readers ticket) at all times, and so the conservators are looking after these irreplacable documents at the county expemse.