According to the Vilna Gaon, the opening line of Kaddish is very disjointed. So much so, that most people are, ostensibly, mispronouncing one of the words.
Grammatically, it would also work, and it would seem to make more intuitive sense, to say:
יִתְגַּדַּל וְיִתְקַדַּשׁ כּרְעוּתֵהּ שְׁמֵהּ רַבָּא בְּעָלְמָא דִּי בְרָא
So, according to the Vilna Gaon, why don't we say it this way (alternatively, why do we say it the way we do)?