It is a mentality that is difference between a creative and business mind.

For the business mind it made great sense to make a sequel, as fans of the first wanted one, the first sold well so a market was there, and it was a guaranteed brand identity to promote.

For the creative mind though it offers no challenge compared to a completely new project. It is one reason many don't make sequels. A designer or artist hates to keep rehashing old ideas, turning out different new versions based around the same old ideas. They want to generate new concepts and ideas. Something completely different and in new directions. If a designer is happy with the concept and outcome if a first game then they have completed the goal they had in mind. And further versions would just be retreading old ideas. I can easily see why using this point of view he didn't want to continue.

The good sequels will always come for design frustration within the first game in a series. Limitations of time constraints, the ideas and designs not fully fulfilled in the first game, or just being unhappy or not fully convinced that the first game delivered everything they wished. And so the designer is eager to make a sequel to try and expand on the first, to show what he really had in his vision for the game.