The answer is that there's lots of reasons, and usually more than one for any given student. The philosopher Michel Foucault would call this state being "overdetermined" -- there's no single root cause we can ferret out that would fix everything. Without consulting hard data sources, here's a list of the top reasons that I see from my personal experience:
- Lack of math skills from high school. Many students simply don't have the requisite skills from high school, or really junior high school (algebra), or in many cases even elementary school (times tables, long division, estimations, converting decimals to percent, etc.). This deep level of deficit is like sand in the engine when trying to learn new math.
- Lack of language skills from high school. What's dawned on me in the last year or so, in the context of applied word problems, is that many students may actually be worse at English than they are at the basic math. Grammar isn't taught anymore, so students can't parse a sentence in detail, can't identify the noun or verb in a sentence, and so forth. This cripples learning the structure of any new language, algebra included.
- Lack of logic skills from high school. No one teaches basic logic, so students can't automatically parse If/Then, And, Or, Not statements, which form critical parts of our mathematical presentations and procedures.
- Lack of study skills or discipline. Almost none of my students do any of the expected homework from our textbook. (On the one hand, I don't collect or award points for homework, so you might say this is unsurprising; but my judgement is that the amount of practice students need greatly exceeds the amount of time I have to mark or assess it.)
- Lack of time to study. Certainly most of our community college students are holding jobs, or caring for children, or supporting parents or other family members. The financial aid system actually requires a full-time course load for benefits; combine that with a full-time job -- really, the equivalent of two 40-hour jobs at once -- and you get a very, very challenging situation. (Side note: In our lowest-level arithmetic classes, I find that work hours are positively correlated with success, but not so in algebra or other classes.)
- Untreated learning disabilities. This would include things like dyslexia, dyscalculia, ADD, etc. All I can do is speculate as to what proportion of remedial students would exhibit such problems if we instituted comprehensive screening. But I suspect that it's quite high. When students are routinely mixing or dropping written symbols, then disaster will result. Unlike other languages, concise math syntax has no redundancies to enable the "you know what I mean" safety net.
- Emotional problems or contempt for the class. I put this last, because it's probably the least common item in my list -- but common enough that it shows up in one or two students in any remedial classroom; and a single such student can irrevocably damage the learning environment for the whole class. Some students who actually know some algebra start the course thinking that it's beneath them, and become regularly combative over anything I ask them to do, sabotaging their own learning and that of others. It's pretty self-destructive, and the pass rate for these kinds of "know-it-all" students seems to be about 50/50.