Complex sentence: However, there are reasons why POST is used even for idempotent requests, notably if the request is very long, due to restrictions on URLs--the query string in the GET method may become very long, especially due to percent-encoding.
Split into simpler sentences: However, there are reasons why POST is used even for idempotent requests, notably if the request is very long. Due to restrictions on URLs, the query string the GET method generates may become very long, especially due to percent-encoding.