No clue really but maybe it's just a packaging problem. A torus takes up more space than a cube.
Toroidal transformer on tube amp
Why are toroids so popular in solid-state amplifiers but so uncommon in tube amplifiers?
They are said to send less EMI and interference to nearby components, among other benefits.
Is there a reason why solid-state designs are "so good" but tube amplifiers aren't?
Why not use them as output transformers?
I've never used EI transformers and have always relied on toroidals.
But, in any case, I've never built a tube amplifier.