What prevents DIY speakers to become the ultimate?

If and when DIY speakers become the "ultimate", they will in all likelihood become "commercial" speakers. The art and business of commercial design is to manufacture a speaker with the cheapest quality parts, make it sound very good (better than DIY for around the same cost) and sell a ton of them. A $300 retail bookshelf (Triangle, PSB, Sound Dynamics, etc.) probably costs the manufacturer around $70 or less to make. But it will sound and look better than anything that the average DIY can make for $300 in parts and their time (at minimum wage). Of course there are exceptions to this, perhaps a Northcreek kit made by a DIY who is also an experienced woodworker.

Most DIYs don't design their speaker projects, but rather use a proven design (Proac clone/ Lowther horn/pipe) or build a kit. This is the safest approach, but don't expect the "ultimate" sound.

I'm interested in making my own speakers as well. I'm not interested in engineering my own- nor am I interested in re-inventing the wheel. I understand that the Thor is a solid design, are there others? Is there a short list of Best DIY kits or designs.

Northcreek notwithstanding, a non-dedicated non-OEM can't buy tight-production tolerance drivers required for superb-imaging, ultranatural response. As alluded above, the best manufactures have custom-made drivers specced within a tight sensitivity window, and even then sometimes pair-match them. (Verity even jeeps a response curve on each midrange for replacement if needed!)
It's a given that Madisound, Solen, etc, get the outer 2/3 or so of a given driver run! I've built a couple of 2 and 3-ways, and once I heard a midrange voicing change of only 1/3dB over 1.5 octaves during a crossover massage, I gave up, knowing that I'd NEVER be able to afford drivers that well matched! This is by far the largest impediment to DIY
ULTRA-high quality production. Though a reseller can sit there and wade through a box of drivers, sorting their sensitivity with a Radio Shack SPL meter, and then selling so-called "matched pairs", that's only going a short way to providing yje standardization required, 'cause you gotta do it with the crossovers next!
Tweeters and caps are notoriously wayward in spectral response. When I heard Verity Audio's midrange, I cried, and now have a couple crates of nice big inductors, caps, resistors, etc., if anyone wants 'em for a dime on the dollar! Spendor, Revel, Snell, and even Boston (!) do a VERY creditable job of pair-matching and building to a target reference with decent precision. Who's going to build us DIY'er some clones to work with? I'll stick to power cords and other non-transducer components....
My hatsoff to those still trying!

Although tempting,I passed on DIY kits...it is hard to justify spending $500-600 on a pair of DIY monitors when you can buy a very good pair of used monitors for that kind of cash...such as the SPendor 3/5,QUad 11L,etc...and be certain of the outcome.. today even $500-600 new will still get you very good sound... w/ alot less headaches...