Why is it "serviceable" but not "servicable"?

I came across this word in the answer provided by Robusto for the question about Thank you.

Because the last e in service is not pronounced, I thought it should be deleted when service is appended by able.

I guess the reason for serviceable may be related to the fact that the i in service is pronounced as [ɪ] but not [aɪ].

However, I am not sure, and I would like to know the reason behind that.

The c is pronounced like k except when it comes before i or e: then it is pronounced like s. In service, c comes before e, so that it is pronounced s. If we add -able, we should normally remove the e, as you said; but then we'd get servicable. Because c before a is pronounced k, the sound of the word would change profoundly; that is undesirable, which is why we add an extra e: serviceable. The same applies to g: change => chang-able => changeable.

Because "servicable" would be pronounced with a hard c.