Why stick to two terms when you are the most popular leader of the most populous country on Earth?
But if you want to get all structural-functional, Xi's power comes from his position as the Chairman of the Chinese Communist Party in a single-party state, not from the fairly low-key position of President, an office with few formal powers. Only in recent years has one person occupied both positions. But in politics, symbolism counts for a lot. Combining the two offices confers an additional aura of authority on the party leader, as does the extended time horizon that comes with the abolition of term limits.
Of course, the rationale for such a move comes down to the promise of continuity of vision and stability of leadership. Authoritarian regimes often point out the incoherence that attends the frequent transitions of power and personality in Western-style democracies, as well as the relative weakness of leaders whose tenure is too short to credibly commit to allies or threaten foes.
In the broader scope of history, China's current rise (or, more accurately, its resurgence after more than a century of decline and turmoil) will be a significant test of the sustainability and performance of two very different forms of governance.