Why did it take two years to close this deal?

Interestingly, Mairitin’s advice to celebrate what seems to be victory the slightly tweaked version of the Comprehensive Agreement of two years ago, for the DUP, is being taken up by the party (even if Brian Feeney argues the reverse is true). However, as Mary Riddel pointed out, just before the last deal was scuppered, the real reasons for the deal are more intensely mundane than either Sinn Fein or the DUP might wish to admit. The sharper question is: why did it take two years, almost exactly, to close this straightforward deal?