Why ships of the line were rebuilt

Was major reconstruction mainly reserved for first-rate ships of the line?

No. Rebuilding was common because funding a rebuild was often easier than obtaining a wholly new ship.

Rebuilding was common rather than confined to first rates. Parliament and naval administration could resist the cost and political visibility of authorizing a wholly new ship. A major rebuild offered a more practical funding route while retaining the identity of a useful vessel. Reconstruction was therefore a routine part of fleet maintenance and procurement rather than an exceptional privilege reserved for the largest ships of the line.