I am aware that the text is not named "Tibetan Book of the Dead" in Tibetan (the Tibetan Title is "Liberation Through Hearing During the Intermediate State"), and that within Tibet's literary corpus, it is not as famous to Tibetans as other texts are; it is a single terma within the Nyingma tradition, which is not even the most powerful school of Tibetan Buddhism.
Nor, from my readings of a few Tibetan texts in English translations, is it the easiest text for a non-Tibetan Buddhist to understand or appreciate. So why was it chosen to be translated so early, and what made the translation so famous?
Really great question. As for why this text was originally given attention by early philologists, I think a good case can be made for pure serendipity-- with such a hue corpus of texts, and relatively little comprehensive understanding of the religion as a whole, certain texts become highlighted more than they might necessarily be in their own emic tradition.
But two factors stand out for this text in particular. First, Evan-Wentz's 1927 translation into English was directly styled after the famous Egyptian Book of the Dead. King Tut's Tomb was just opened in 1922, and the world was a bit obsessed with ancient Egypt at the time. By styling the book after the Egyptian, it tapped into to some of this Egyptian fervor.
Second is probably Timothy Leary, Ralph Metzner, and Richard Alpert's 1964 book The Psychedelic Experience, which took the broad outline of the Bardo Thodol ("The Tibetan Book of the Dead") and gave an interpretation of it as a guide for psychedelic tripping. All three were hugely influential in the counterculture, and eventually mainstream culture as well, and their highlighting of the text cemented it even further into the mainstream psyche. This was also right after the Dalai Lama fled from Tibet to India in 1959, and the larger outside world had its first real contact with Tibetan refugees. So, this contact with Tibetans, a growing interest in Tibetan Buddhism because of this, the broader turn towards "Eastern" religions as part of the 1960's coutnercultre movement, and Timothy Leary and Richard Albert's (later called Ram Dass) "translation"-- I think interpretation is better here-- propelled the book even more into the mainsream.