Mainly because it's easier to renew a medical certificate than to obtain one under the 'initial issue' procedures. If you had paid a fortune for commercial flying training and then discovered that you couldn't renew your Class 1 medical certificate........?
The rules are vague and I don't know what CAA policy is, but the following extract from JAR suggests to me that the CAA could ask you to undergo a full 'initial issue' examination at Gatwick to renew a lapsed Class 1 medical :-
"If a licence holder allows his certificate to expire by more than 90 days but less than two years, renewal shall require the prescribed standard or extended examination to be performed at an AMC, or by an AME at the discretion of the AMS"
Good luch whatever you do!