Because they do a check on you before the test,,,RCMP,,,Canadian secret agency check, border agency check ,,,etc
they wait for a clearance report from all this peoples ...
They have also ,,,big number of files ( not only people in the forums

) imagine that all people who come to Canada by immigration ,,,will apply for citizenship eventually,,,,that means at least 350 000 application a year....+ the accumulation if there is previous delay
add to all that ,,,the citizenship fraud that happens and makes the procedure more severe
Hope all that will answer your question