They often just had no interest.

As you say, Christian theologians could have found Jews to teach them Hebrew at many points (though there have been those who have considered it against Jewish law to teach Hebrew to gentiles). It wouldn't have always actually been that easy, as Jews were expelled from large chunks of Western Europe, for example, for centuries, but it still would have been possible. But they rarely had an interest. Why should they? Ecclesia had already replaced Synagoga, the Jews were inferior and the Christians had eclipsed them, and Hebrew was the language of the Jews and their Bible.

As you note, that didn't mean that priests who were interested in Hebrew didn't exist. There are a number of cases noted in the 13th/14th centuries or so, as you mentioned, in which Christian theologians noted some knowledge of Hebrew and Jewish scholarship, whether in a benign or even positive sense (the preeminent Jewish commentator on the Bible, Rashi/Rabbi Shlomo Yitzchaki, was cited by Nicholas of Lyra as "Rabbi Salomon Hebraeus") or with more negative connotations (according to writings of several hundred years later, the English prior Gregory of Huntingdon, known to have known Hebrew, eagerly bought at auction the Hebrew books which had been the property of several synagogues which laid empty as the result of the expulsion of the Jews from England in 1290). But overall, it was just not something that interested Catholic scholars particularly. They had their Bible in Latin and had little desire to know more about the Bible in Hebrew, the language of a despised people.

That said, there were also a number of Catholic priests at this time who knew Hebrew and used this knowledge to their advantage- but they were Jewish converts to Christianity, many of whom went into the Church and used their knowledge not just of the Hebrew Bible but of the Talmud in order to assist the Church in its missionizing efforts. Famously, apostate priests such as Pablo Christiani and Nicholas Donin forced leading rabbis in 13th century Spain and France, respectively, into debates with the object of proving the truth and supremacy of Christianity over Judaism, with the long-term goal of achieving conversions. These converts from Judaism also used their knowledge of Hebrew to read Hebrew translations of Greek works, by way of Arabic, and translate them into Latin; often they were joined in this by born-and-died Jews who used their knowledge of many languages as a way to make a living.

The question is more what changed in the 15th century or so, and the answer is essentially the humanist movement, leading into the Protestant Reformation. For example, Johannes Reuchlin was a Catholic lawyer and scholar of Hebrew and kabbalah who attempted to start a movement in favor of teaching and publishing Hebrew language literature as part of the new humanist movement, though he did not receive a great deal of support from the mainstream Catholic establishment. Reuchlin had been taught Hebrew by Ovadia Sforno, a well known Italian rabbi and exegete, and was considered by the eminent Jewish historian Gershom Scholem to have been the father of academic Jewish studies. Erasmus of Rotterdam, who was envisioning humanist reform of the Catholic Church, saw Hebrew as a way of going back to the roots of Christianity, without being interested in Hebrew- and certainly not in Jews (as I'll discuss in a minute).

With the Protestant Reformation came an increase in interest in Hebraism, as Protestants began to use the Hebrew Bible as a foundation for their attacks on Catholicism- and Catholics in turn learned Hebrew in order to counter these attacks. While the initial generation of Christian Hebraists were taught by Jews, out of necessity if nothing else, soon Hebrew began to be taught as a part of a well-rounded classical education. While Jews had been printing books in Hebrew since 1475 and Christian printers had been printing Hebrew books as a sideline for nearly as long, the demand for such books became even greater. As time went on and a second and third generation of Christian Hebraists came into being, Christians learning Hebrew went beyond merely engaging with the Hebrew Bible into learning about Judaism in and of itself- I wrote a piece discussing this here. There was a particular interest in kabbalah, which Christians saw as ancient teachings which were applicable to Christianity.

But one thing that's really important, especially since you mention antisemitism in your OP- Christian Hebraism did NOT mean philosemitism (or, if it did, it was philosemitism with all of its negative connotations). The mere fact that a Christian scholar, Catholic or Protestant, was interested in Hebrew or Judaism for his or her (there were a number of female Christian Hebraists) own edification did not mean that the scholar particularly liked Jews. For every Reuchlin, who started off attempting to coopt Jewish teachings but ended up a friend and defender of Jews against anti-Jewish campaigns, including one which would have caused the destruction of all Jewish books in the Holy Roman Empire, there was an Erasmus (who didn't have much good to say about Jews in his private correspondence) or a Johannes Pfefferkorn (a convert from Judaism who wrote scathingly against it) or a Johannes Buxtorf the Elder (whose tremendous amount of study of Judaism led to a book about it which was filled with negative and antisemitic statements and stereotypes). As a general rule, this interest in Hebrew came from a supersessionist perspective, in which Judaism was by definition both inferior to and replaced by Christianity, and even the most philosemitic Christian Hebraist was approaching the subject from a fundamentally condescending perspective. Later, in the 17th century, millenarianism became a relevant factor as well, with Christians becoming interested in Jews because of their potential eventual role in the Second Coming. Christian Hebraism was a movement that was entirely premised on appropriation of Judaism and Jewish language and culture for Christian purposes, even at its best.