Because the context of Matthew 24 was for his generation, the generation that crucified the their Messiah.
"Truly, I say to you, this generation will not pass away until all these things take place." Matt 24:34
That is why we should always be prepared, not because of the fear of end times but because He comes life a thief in the night (Thess 5:2) and our short lives can be taken anytime.
The Book of Revelations is a very important book but many eschatological beliefs have been created out of this. The problem with these apocalyptic end times beliefs is that people put their personal lives right into scripture but when looking at the entire context of the Book of Revelations, it becomes clear that John's visions of salvation and judgement were for the people of 'HIS' time and not 'OURS'. The book does show correlations to what we are experiencing today and the same can be said on morals and ethics but think about how many denominations have given a date for end time prophecies only to find disappointed the following day; but followers/believers would have sold their assets ending up flat broke and for what? a bad interpretation?