Since so few Catholics avail themselves of something that is supposedly being guaranteed to work, we would need to assume a possible reason for this:
1) Weak cases are screened out by the Pastor's of Parishes.
2) 85%+ of divorced Catholics reject modern annullment practice because they hold the indisolubility of marriage at a much higher level than does the Church. (Yet many of these same people remarry outside the Church and continue frequenting the sacraments.)
3) 85%+ of divorced Catholics think annullments are just a big hassle and a joke that they don't need to deal with, because they are already divorced, and that first Church marriage was just to make the folks happy. Plus God understands our hearts, yadda, yadda, yadda ...
I suggest (1) is probably likely for much of it, with a smattering of (2) and (3)'s.
Do you have some alternate theories for why so few Catholic divorcees come before the tribunal? Lets assume all the annullments given out since 1968 were only to Catholics. If so, only 2 million of 6 million divorced Catholic marriages went before a tribunal and were dissolved, leaving 4 million unions in marital limbo.
There must be something at work keeping those 4 millions back from a "guaranteed" process.