But reader-comments there note that Rabbi Saul Lieberman, also of JTS, has had his commentaries on the Tosefta (published by JTS itself, no less!) avidly studied even by Haredim! So the readers suggest that whereas Lieberman's works are indispensable, and so scholars overlook his ties to JTS, Heschel's theological orientation and Hassidc mystical tendencies make him irrelevant to Lithuanian-style Talmudists.
I'm wondering if perhaps a key factor is that Heschel's theology of "G-d in search of man" sounds far too much like Rav Hirsch's teaching that the Torah is an anthropology and not a theology. (Heschel is certainly indebted to Hirsch; see the translator's appendix to Rabbi Dr. Leo Adler's The Biblical View of Man, Urim Publications.) Having G-d seek man implies too many expectations of man by G-d, too much responsibility on man's part. It is much too humanistic, too perspicacious and audacious. It is simply too controversial to put so much power and responsibility in man's hands; it is much more comfortable to let spirituality and mysticism be a salve for the conscience; do a few theurgic mitzvot, engage in some theosophical speculation, and you're good by G-d.
Indeed, Gelman quotes Heschel as saying,
The Bible is an answer to the question, What does God require of Man? But to modern man, this question is surpassed by another one, namely, What does man demand of God? ... Absorbed in the struggle for emancipation of the individual we have concentrated our attention upon the idea of human rights and overlooked the importance of human obligations. [Emphasis added - M. M.]
If G-d is in search of man, it means He is seeking out man to give him responsibility, expecting something meaningful of him. By contrast, if man searches for G-d, then man is seeking to satisfy his own needs and desires, and he accepts religion insofar as it suits and satisfies his base sensual desires. Compare Rabbi Emanuel Rackman, describing Rabbi Soloveitchik, quoting from here:
Soloveitchik regards as altogether too simple the popular notion of religious experience as one preeminently pleasing and soothing-a stream of delight and relaxation and an asylum from the frustrations of life. This conception of religion Rabbi Soloveichik deems a fraud, the result of a surrender on the part of religious thinkers to the desire of the mass of men to lose themselves in states of bliss. It also echoes Rousseau in his flight from reason, and much subsequent romanticist thought. Religion's invitation has been misinterpreted to say: "If thou cravest peace, if thou cravest integration, make the leap of faith." In the flight from reason and the rejection of objective truth, Rabbi Soloveichik sees the cause of the moral deterioration of contemporary man. He would prefer to see religion wedded to a cold objectivity and rationality, even though faith and reason may at times appear to conflict with one another, rather than derive religion from man's instinctual longings.
It is highly significant that Rabbi Soloveitchik prefers a cold rational empiricism to a warm spiritual narcissism. But this is contrary to our natural inclination, in which we wish to turn to G-d and authority figures for satisfaction of our desires, even as we abdicate our own free choice and critical faculties into their trust, to free ourselves of that burden. Thus, we pay only mere lip-service to Rambam's teaching in Hilkhot Avodah Zara that Avraham Avinu found G-d via reason and intellect; as Rabbi Yom Tov Schwarz (in Eyes to See, Urim) and Professor Menachem Kellner (cf. here) both point out, Rambam's explanation implies Avraham found G-d by going against authority and tradition, and instead using his critical reasoning faculties, accepting only that which was empirically evident. This is not a message Orthodoxy is comfortable with.