It's not that REM initiates only after 5/6 hours... let me explain..

When you first fall asleep, you have a very very brief dreamlike state... HI mostly. Then you hit deep sleep. You stay like that for almost 90 minutes before you come up to dream for only a few seconds.

Then it repeats. But this time, you have more than a few seconds of dreams.

Then it repeats again. Each time more dream time.

After this number of cycles (4.5 hours), you have more dream time than deep sleep.
This means it's easier to hit a lucid dream because (1) there's more dreams and (2) you did a WBTB which awakened your consciousness a bit