Daniel Klein argues in a Minding the Campus essay that liberals dominate academia because of ideological bias in hiring rather than conservatives being afraid that people will "giggle" at their ridiculous jobs in academia. Klein is right to reject the giggle theory, but for all the wrong reasons. Klein cites his evidence that non-academics who are members of disciplinary organizations are slightly more likely to be Republicans than academic members to be the "smoking gun" evidence.
What Klein calls "smoking gun" evidence of ideological exclusion is, in fact, not very persuasive, as I note in my Patriotic Correctness book. Klein somehow thinks that work in the private sector or conservative think tanks provides far less income and inferior jobs in every way, and these jobs are only taken by those shunned by academia.
There's not the slightest evidence to support such an assumption. For example, most professors would jump at a chance to become a highly-paid fellow at the Hoover Institution, freed from the demands of teaching and academic research.
Klein's smoking gun could easily be evidence of the opposite effect: that liberals are more often excluded from private sector jobs than conservatives. It could also be evidence that people in academia become more liberal and people in the private sector become more conservative over time due to their colleagues. But to assume that ideological bias in hiring as the sole explanation cannot be justified.
I agree with Klein that the giggle theory is garbage. To answer the question, we need to look at the fact that among students entering college, both liberals and conservatives express a strong desire to teach. Yet conservatives rarely enter either academia or K-12 teaching. Since the barriers to jobs in K-12 teaching are much, much lower with no serious allegation of systematic ideological bias in hiring that I've ever heard, it suggests that a common answer explains the lack of conservatives in all teaching fields. Money appears to be the key answer as far as I can see.