Why is the phrase 'cross the T's and dot the I's' not 'cross the X's and dot the J's'?

Pray forgive this transgression, but it seems relevant:

I:
I is the first letter of the alphabet, the first
word of the language, the first thought of
the mind, the first object of affection. In
grammar it is a pronoun of the first person
and singular number. Its plural is said to
be We, but how there can be more than one
myself is doubtless clearer to the grammarians
than it is to the author of this incomparable
dictionary. Conception of two myselves
is difficult, but fine. The frank yet
graceful use of "I" distinguishes a good
writer from a bad; the latter carries it with
the manner of a thief trying to cloak his
loot.

X:
X in our alphabet being a needless letter has
an added invincibility to the attacks of the
spelling reformers, and like them, will
doubtless last as long as the language. X is
the sacred symbol of ten dollars, and in such
words as Xmas, Xn, etc., stands for Christ,
not, as is popularly supposed, because it
represents a cross, but because the corresponding
letter in the Greek alphabet is the
initial of his name - Χριστος. If it represented
a cross it would stand for St. Andrew,
who "testified" upon one of that shape. In
the algebra of psychology x stands for
Woman's mind. Words beginning with X
are Grecian and will not be defined in this
standard English dictionary.