Rashi gives this as the reason of the prohibition: For then the
restoration of the Jews to their own land will take place, so
that the denarius paid for a field in a foreign land would be
money thrown away.
Four thousand two hundred and ninety-one years after the creation of the
world the wars of the dragons and the wars of Gog and Magog will cease,
and the rest of the time will be the days of the Messiah; and the Holy
One--blessed be He!--will not renew His world till after seven thousand
years.... Rabbi Jonathan said, "May the bones of those who compute the
latter days (when the Messiah shall appear) be blown; for some say,
'Because the time (of Messiah) has come and Himself has not, therefore
He will never come!' But wait thou for Him, as it is said (Hab. ii. 3),
'Though He tarry, wait for Him.' Perhaps you will say, 'We wait, but He
does not wait;' learn rather to say (Isa. xxx. 18), 'And therefore will
the Lord wait, that He may be gracious unto you; and therefore will He
be exalted, that He may have mercy upon you.'"
THE TALMUD, _Sanhedrin_, fol. 97, col. 2.