Because that is the ONLY scripture Isa.43:10-12 in the whole bible that they feel gives them validation for calling themselves Jehovah`s Witnesses and being Gods chosen people among Christendoms religions.
therefore they alone have the "Truth" so they believe.
Of course they ignore so many other facts.
1.The G.B. admitted in their publication the Aid book that the name Jehovah was not the correct pronunciation of Gods name however it was accepted as such because it was the most used and common placed in Christendom.
2.The actual name "Jehovah" did not appear in literature until the 13th Century when a Spanish monk used the Latinised version of the Hebrew Tetragrammaton ,the four Hebrew letters that represented Gods name in the Hebrew bible the Old Testament and added vowel `s to it ,coming up with JHVH and adding E ,O,and A and coming up with JEHOVAH which was then used throughout Christendom from the thirteenth Century on which is attestable to in many Churches throughout Europe and elsewhere on their buildings with the name Jehovah in stone.
3. After that scripture in Isa, was written hundreds of years before Jesus Christ came on the scene the Jews never did identify themselves as Jehovah`s Witnesses did they and Jesus when he came on the scene never berated them for not identifying themselves as Jehovah`s Witnesses .
So that was never the intent otherwise J.C.would have done so.
4.The ancient Hebrew language never had a J sounding letter that is why some Bible translations do not use J in their translations so instead of Josiah for example they might use y`osiah or instead of Jesus ,Yesus etc.
So the modern version of the more likely version of Gods name that the Jehovah`s Witnesses recognized in the Aid book was that Yahweh or some such was more likely accurate however Jehovah was preferred because it was more popular ?
Popular among whom ? they claimed that Christendom was hiding the name of God from everybody so who was the name more popular with ?
Why Christendoms Scholars of course whom the claimed were Apostates of Christianity .
And what did Jehovah`s Witnesses do ? They adopted Christendoms preferred name for God Jehovah .