The reason cannot be because if it's the only reason.
The reason is always that.
When you say "The reason spotted octopi are crotchety is because of their preference for starfish," you are saying "Spotted octopi's preference for starfish causes the reason they are crotchety."
The right thing to say is "The reason spotted octopi are crotchety is that they prefer starfish." or "Spotted octopi are crotchety because they prefer starfish."
The word "reason" when used in this context implies causality. You end up with a confusing double causality when your reasons are because of something, unless you're intentionally trying to set up a chain of reasoning, in which case:
The reason spotted octopi are crotchety is because of their preference for starfish. That is to say, preference for starfish leads to the reason for crotchety spotted octopi. The real reason spotted octopi are crotchety is that their tentacles tend to wilt in aubergine-coloured water. However, this reason is conditional upon preference for starfish.
What you end up with is the following logical chain:
But all you ever wanted to say was that preference for starfish causes crotchetiness in spotted octopi! Don't complicate it. The reason is that. Not because. That.
I just see so many people say "the reason is because" and it inevitably throws me off and causes me to spontaneously combust. When I go back to reading I find that I've been thrown out of the story/off the point of an essay. Please. Really. The reason is that. Embrace it, love it.
Next time you're tempted to write "the reason is because," please, think of the octopi!
**I have no idea if spotted octopi actually exist, whether they prefer starfish or electric eels, and whether aubergine-coloured water has any effect whatsoever on the crotchetiness of octopi. Also, if you can tell me how to properly spell the possessive form of "octopi" I shall be grateful. (ETA: thanks, kaikias)
ETA2: Courtesy of pen_and_umbra: To be extra anal-retentive with a side of philological snobbery, the correct plural for "octopus" (from Greek, "eight-footed") is "octopodes". The plural can also be anglicised to "octopuses" but applying the Latinate plural suffix "-pi" to form the odd polyglot plural "octopi" is patently wrong. It now appears in dictionaries, too, but that doesn't make it less jarring to anyone who's done hard time in the classical linguistics department.