Why did the Byzantines call Western Europeans beef-eaters?

Because Byzantines did not eat beef as often as Western Europeans did.

See Karen Carr’s answer to What was the basic diet like in the Byzantine era (circa 530) under Emperor Justinian and Empress Theoradora?

They occasionally ate lamb and mutton, chicken, and pork; rarely beef.

Or the Oxford Dictionary of Byzantium, s.v. meat:

The most popular kind of meat was lamb … Goat meat is mentioned, among others, by Liutprand of Cremona, who did not enjoy the “fat goat” served at the imperial court in Constntinople. Pork was considered a coarse food, whereas the chine of beef appears in Niketas Choniates as a staple of imperial banquets and of the Crusaders.

Or An Entertaining Tale of Quadrupeds, which I have cotranslated: the Sheep, Pig, Goat and Rabbit each boast of how tasty their meats are; the Buffalo only mentions its cheese, and the Ox does not mention either meat or dairy products: he only mentions how his horn is used to polish books, and his penis is used to make whips.

Why beef would be avoided is a question I’m not fully across. If the Ancient Greeks could come up with the Hecatomb, they certainly had neither a shortage of beef, nor a reluctance to eat it. Greeks do eat beef now—though more veal than beef, and much more lamb than either.

The related question, why do I keep referring to Western Europeans from a Greek perspective on Quora as “beef-eaters”, is because Niketas Choniates memorably used the phrase in his account of the sack of Constantinople:

Οἱ δ’ ἀγκροῖκοι καὶ ἀγελαῖοι ἐπεκερτόμουν μᾶλλον τοῖς ἐκ Βυζαντίου ἡμῖν καὶ τὴν ἐν πτωχείᾳ καὶ γυμνότητι κακουχίαν ἰσοπολιτείαν ἀφρόνως ὠνόμαζον, οὐ τοῖς τῶν πέλας κακοῖς παιδευόμενοι. πολλοὶ δὲ καὶ ἀνομίαν ὑπολαμβάνοντες „εὐλογητὸς Κύριος, ὅτι πεπλουτήκαμεν“ ἔλεγον, ὀλίγου τὰς τῶν συμφυλετῶν οὐσίας ἀποδιδομένας ὠνούμενοι. οὐ γάρ πω βουθοίνας εἰσῳκίσαντο Λατίνους, καὶ εἴδοσαν, ὅπως μὲν τὸν οἶνον ἄκρατον ὁμοῦ καὶ ζωρότερον ὥσπερ καὶ τὸν χόλον ἀκέραστον χέουσιν, ὅπως δὲ Ῥωμαίοις ἐν ὑπερηφανίᾳ καὶ ἐξουδενώσει προσφέρονται.

The peasants and common riff-raff jeered at those of us from Byzantium and were thick-headed enough to call our miserable poverty and nakedness equality, without learning anything from the evils perpetrated at their doorstep. Many were only too happy to accept this outrage, saying “Blessed be the Lord that we have grown rich”, and buying up for next to nothing the property that their fellow-countrymen were forced to offer for sale, for they had not yet had much to do with the beef-eating Latins and they did not know that they served a wine as pure and unmixed as unadulterated bile, nor that they would treat the Romans with utter contempt.