What prevents DIY speakers to become the ultimate?

One of the exceptions is the Ellis Audio 1801, a DIY two-way monitor that can compete with the best of 'em.
http://www.ellisaudio.com

I have helped out an electrical/audio engineer friend on a
speaker project that turned out very well. We did not have access to an anochoic chamber and that is a serious handycap. But he had LEAP and another testing/modeling programs and went into the backyard to do the tests. He also spent two years blowing off college focusing on speaker design. It requires tons and tons of dedication and some natural talent. My opinion: for most people it would be better to build a pre-designed kit, you get most of the economic advantage and it's still fun and a learning experience. The other thing you could do is try to copy a design, more difficult but possible. Regular guys just don't have the access to equipment that the pro's do, and experince does matter! But who knows maybe your the next great speaker designer!!!! Someone will be?

In my experience, the one thing I would wish for is obtaining drivers which were matched very closely. For example, if a manufacturer specs a +/- tolerance of 1.5 dB, you very well could end up with 2 drivers which are up to 3 dB apart. Thankfully, North Creek offers perfect matched drivers. Have yet to try any.

Apart from that, I must say that while I agree that a DIY cannot achieve the ultimate, how many loudspeaker manufacturers do?

Most DIY designs can compete with most audiophile products. Start with the wisdom of those who came before us. Some simple, cheap tips that people a lot smarter than me have always stressed to me:
1)Remember that mid level drivers in a good box normally sound better than great drivers in a mid level box.
2) Instead of going crazy getting the "best" drivers, think about doing the little things that add up to good sound. - Use really good glue, lay a bead of glue or caulk inside all of the cabinet's outside joints.
- Always use a gasket in the driver/cabinet interface
- Don't put a lot of money in wire. Use good copper(cheaper than you think), and stay away from the cable companies
3) Crossovers are important...
- Despite what a lot of people say, it's harder to mess up a simple topology than it is to get a complicated one right. Keep it simple
- Series crossovers are very forgiving of the tolerances one finds in drivers, caps, and coils. Series crossovers also seem to have a coherence between drivers that is hard to match, glossing over a lot of what you or the component manufacturers did wrong
- If you don't feel comfortable designing a crossover, use a designer's. Nothing flatters them more

And finally, we are normally more critical of ourselves and our work than we are of that of others. When we do a project, we know the mistakes we make, and dwell on them always. When we buy something from others, we assume it is perfectly designed and put together. Not always so.

Wasn't most every speaker company once a DIY venture? I don't think Jim Thiel, etal, slapped their label on ready-made stuff and proclaimed themselves a "commercial" company.

I have been a DIY speaker builder for 20 plus years. I even worked at a small speaker kit company in college, making all of their cabinets. After college I kept building speakers and buying the best parts I could get my hands on. I am a trained cabinet maker and have tried just about all of the differnet types of cabinets and construction styles. I can build a very good "box" Many of them sounded good but none of them great.
Then a few years ago I build a set of Northcreek Rythem's and the whole DIY thing changed in a HUGE way for me. As it turns out the crossover was the key to way my speakers never sounding great. Granted Northcreek cabinet designs are excellent but their crossovers do set them appart.
I have now built many different Northcreek speakers and they all sound GREAT. I have a friend who owns a set of ProAc 3.8's and my Rythem sig. much better (I hope he doesn't read this) . I have another friend who has also built Northcreek speakers after hearing mine.
Also just to give you a refernce point my system is:
Sony AX777es CD/sacd, Ayre V1 & V3 amps, Arc LS25 pre, EAD Sig, processor and Cardas ref. cables.