I never noticed that its actually the flag colors!
It's also available in every country with an Ikea, which is all of europe, north america, oceania and large parts of asia.
In addition the colours matching the trans pride flag, I’ve heard people suggest that it’s because trans people (like sharks) are significantly less dangerous than the media portrays
This is exactly the reason, it became a meme (like Fallout New Vegas becoming a thing in the trans community before it) and people ran with it. People have retroactively said that it's because blåhaj is trans pride colors but it wasn't the initial reason.
Its soft and cuddly and, to some extent, the same colors as the trans flag. It’s a much bigger thing among transfems than transmascs, and I’ve heard that part of its popularity is that it lets transfems feel soft and cuddly, when previously they’ve spent their whole lives under male social expectations where that’s less encouraged. It being a meme is a fun way to celebrate the expressing of a buried part of yourself.
Also, IKEA ran an advertisement in support of gay marriage, using the shark. I forget, though, if that came before or after it becoming a trans symbol.