Because all the evidence in fulfillment of Bible prophecy indicates this corrupt system is due to end in a few years. Of the generation that observed the beginning of the “last days” in 1914, Jesus foretold, “ This generation will by no means pass away until all these things occur.” – Matt 24:34.
From Rutherford's time the Watchtower heavily overused the word 'evidence'. A good way to manipulate a naive and susceptible mind to believe your statement. Imagine that, "...all the evidence...". Now let's add some more weight to this "idea".
" Of the generation that observed the beginning of the “last days” in 1914, Jesus foretold,.."
What right thinking person is going to argue with that statement? This is the very generation that Jesus was foretelling about. No need to think about it further.
"...ALL THE EVIDENCE ...INDICATES...".
Strange language. The word 'indicate' has the thought of 'kinda leaning in that direction', HOWEVER, 'evidence' proves it beyond any doubt. It seems that the Watchtower writers were kind of leaving themselves an out. " We only said it 'indicates ... the system was due to end.. We did not say it PROVES IT."