I think that this topic suffers from a lot of vague questions and false assumptions.
1. The question is unclear what sort of “Jewish writers” OP is referring to.
2. The question is unclear as to what sort of quality OP is referring to.
3. The question (and many of the answers) seems to think that writing is something that you are either good at or not good at. But that is untrue. In addition to innate talent, good writing is the product of education, practice, and work.
4. Pursuant to the above, there are lots of reasons why frum books are not as “good” as secular books. Frum people writing for the frum market are unlikely to have a literary education. Frum people writing for a frum audience are unlikely to be able to generate the type of income that the level of work would require. The very best-selling frum books sell 20,000 copies, and those are a very specific type of book – populist fiction. If a frum person wanted to write the “great frum American novel” it would take him thousands of hours and he would lose money on it. It’s not a coincidence that even the best-sellers (Yair Weinstock type novels) are written mostly by Israelis – they find it worthwhile to put in the work for less money than an American would require.
5. Even if everything I’ve written above is false, it would be easy to understand why frum writers don’t write in the secular market. Even the most talented writers need a ton of luck to get noticed and make a living off their writing. If among 100 published frum writers, none of them happened to get lucky, it would not be anomalous at all.