Toroidal transformer on tube amp

No clue really but maybe it's just a packaging problem. A torus takes up more space than a cube. 

EI core transformers have inherent air gaps where energy in the magnetic field is stored, toroid transformers do not. If you get any DC bias on a toroid transformer, the inductance will change by a large amount. That’s bad. It will also stay magnetized after the DC bias is removed continuing to effect the inductance. The EI core transformer has more stable properties. I believe the EI is also more linear over a wider range due to the air gap, but I may be mistaken on that. I had the same question many years ago. I get the theory, but have not spent a lot of time thinking about it. I am sure others could weigh in in more detail.

Torroids are smaller and lighter, which is the main reason they’re used is SS gear. Using a torroid as a mains transformer on a tube amp is okay, but it has a wider footprint, so it’s a matter of allocating real estate that usually takes priority when deciding to use one. Torroids are not used as output transformers for one main reason: any imbalance in tube plate current or gain will alter the a.c. operating point and create lots of distortion. Also, it would have to be bigger in size to guard against saturation at low frequencies. But if every tube was identical and wore out exactly the same, then torroids would be far better output transformers.