The reason is that spiritual poverty, in a certain sense, is not a good thing at all.
To be spiritually poor, in the first dimension of the word, is to be a sinner. This is not a good thing. Jesus isn’t talking merely about humble people here; he is talking about sinners.
But we know that being a sinner in itself is not at all something blessed. So there is certainly more going on here. This is the second dimension of spiritual poverty — namely, recognizing and acknowledging that you are a sinner.
The church in Laodicea had this first dimension of spiritual poverty, but not the second: “You say, I am rich, I have prospered, and I need nothing, not realizing that you are wretched, pitiable, poor, blind, and naked” (Revelation 3:17). They were spiritual poor in the objective sense (they were sinners), but not in the subjective sense (they didn’t recognize their spiritual poverty, but instead thought their material wealth made them rich and independent). That is not blessed.
When Jesus says, then, “blessed are the poor in spirit,” he means: “blessed are those who subjectively recognize what is objectively true about themselves — namely, that they are spiritually bankrupt and sinful apart from My grace.”
This state is blessed, because this is precisely the reason came: not to call the “righteous,” but to call sinners to repentance (Matthew 9:13). And if we do not recognize that we are sinful — if we do not acknowledge our spiritual bankruptcy in ourselves — we cannot heed his call to repent and come to him to receive true spiritual riches.