Taiwan's semi-presidential system was established in 1997, but demanded an overhaul in 2015. The constitutional reform efforts in both years began with the parliament delivering its proposal for revision by a three-quarters majority for approval at the next stage, and in both years the reform efforts were dominated by Taiwan's two largest political parties, the Kuomintang and the Democratic Progressive Party. Although the body responsible for approval in the second stage of passage was changed in 2005 from the National Assembly to the public, there was a significant similarity in 1997 and 2015 at the legislative stage. Why, therefore, did Taiwan's legislators adopt constitutional revisions in 1997 but fail to do so in 2015? Taiwan's two largest parties held diverse views concerning whether the appointment of the premier should be confirmed by the parliament, yet this difference existed at the time of both reform efforts. The real puzzle is why the consensus reached in 2015 did not result in constitutional amendments. This essay proposes that whether a constitutional reform can change the status quo is strongly affected by the separability of issue preferences. Accordingly, what must be compared is the package of issues rather than the individual issues themselves. Most likely, the decline of authoritarianism and growing consolidation of democratization has reduced room for party cooperation.