For decades, it has been a common practice to cover existing wood shingle roofs with asphalt (or "composition") shingles when the house needed a new roof. It wasn't perfect, but it worked. A few years ago, after many storm related insurance claims, the major insurance companies made a business decision not to write new policies for houses that had asphalt shingles over wood shingles. It doesn't matter what kind of shape the roof is in.
Two, and sometimes three, layers of asphalt shingles are fine. But, not asphalt over wood. Why? For the insurance company it just costs so much more in labor and materials to replace such a roof system if you were to make a claim in the future. A wood roof has gaps underneath the shingles to allow the roof to "breathe". Asphalt roofs instead need solid decking to go under the felt and shingles. So the asphalt-over-wood roof needs to be stripped to the bare rafters and started from scratch. That's where the expense comes in.