The most popular breed at mid to upper levles - 3/4 TB, 1/4 Draft. If bred properly. Thoroughbreds still run at all levlels - but they aren't as popular as they used to be.
The reason being is because they are too "fine boned" to beable to survive in this very demanding sport. Same reason why you don't see 1/2 drafts or full drafts at Mid to Upper Levels - too heavy to survive in this demanding sport.
So because the TB is too "fine boned" they found that the majority of the % out there would end up with joint and bone issues, resulting in retirement etc, etc.
So they threw a breed that had bone into the mix.
The 3/4 TB still has the best qualities a TB can offer. Heart. Speed. Stamina. But the 1/4 Draft mixed into that equation - now gave the horse more bone to beable to withstand the demanding impaction from what jumping creates. AKA Sport Horses. Irish Sport Horses
are VERY prominant in this sport. But don't forget they are 3/4 TB and 1/4 Irish Draught - aka 3/4 TB 1/4 Draft.