Partly Pipe, as far as we all know, but also just the overall improvements in horse care, SS. There are now so many different feeds for every stage of their life, from foal to retirement; veterinary products abound for every possible twinge and twitch; therapies previously only known to expensive spas or gyms for rich people are now becoming almost commonplace (swimming, salt baths, heat lamps, ultrasound, Reiki, Shiatsu, heated indoor treadmills, a variety of cooling or heating gels and pads for legs and backs, etc., etc.); veterinary care in terms of surgeries, endoscopy, and drugs; vitamin and other supplements for feeds; and finer-tuned early schooling, which some yards are borrowing from the French, or by buying ready-schooled French hurdlers and chasers.
There are also now many more artificial aids to maximising a horse's performance than previously used or even allowed. We think nothing of shadow rolls/sheepskin nosebands (they were first used in the US in the 1950s, I think), but now we have the regular use of tongue-ties, pressure nosebands, ear defenders, cheekpieces instead of just old-style blinkers, cup visors... on we go!
So, overall, the days when BROWN JACK jumped for fun and stunned racegoers are really ancient history in more ways than one. Horses is his day were regularly bar-fired, let alone pin-fired, proactively to strengthen tendons. Jockeys mostly rode like sacks on their backs, taken from the hunting seat, not nearly up with the Flat jockeys in stirrup length; the horses had their heads hauled off by enthusiastic but not race-tuned amateurs; they would probably be out hunting with the Quantocks one day and chucked into a race a few days' later with no real race prep at all.
All horses now, Flat or NH, are treated very much more precisely and individually as to their food/supplement needs, they're not all ridden in a one-size-fits-all manner, and their ancestors would be astounded at such innovations as the stuffed hurdle, a treadmill, the water therapy pool, and as for needing ear defenders to deaden the roars of the unrestrained, unsecured crowds mobbing them... !