The keywords that are showing as inactive are deemed as such by a complex number of factors including your sites "quality score" (which is more like a measure of google's views on your business model), your click-through-rates, a keyword's history of performance (and this may extend beyond the keywords performance for you personally), and probably the level of competition for said keyword.
Read more posts in this forum to learn more about the inactive keywords debacle.
And one thing to note: inactive keywords are still active for the content network, so if you can't afford 'em, you can still keep them around to trigger your ad on the content side of things.
Good Luck!