Because size (and complexity) directly correlate with cost. So unless fusion can enable cost reduction elsewhere, if the nuclear island is inherently larger, more complex, and hence more expensive than in a fission plant, power from it will be more expensive than from a fission plant. And in that case, why would any utility want one? New and risky (in the sense of having a significant chance of not working as well as hoped) technologies like fusion will be adopted only if they are significantly less expensive than more proven alternatives.

The size and complexity also directly affect reliability. There is more to go wrong in a fusion reactor than in a fission reactor, and repairing anything there will be difficult because hands-on work will be impossible.