Less port flow and/or back wave info I imagine. I've noticed you can place a TL design up against a wall without smearing the bass, which cannot be said for bass reflex. Each design is different, so I imagine some will be better than others, but the TL cabinet seems to be more user friendly regarding room modes and boundaries (probably more of the latter). I'm not real familiar with the technically specifics behind the two designs, so I'm just speaking based on my own findings and others as well. I probably should have said room boundaries rather than modes, so I edited my post for you.
I found this post by Paul K. that might help explain things...??
"Unlike vented systems, a TL will have stuffing throughout part of its length, not just partial stuffing or lined walls, and this aids in limiting, if not preventing, internal reflections from coming back through the driver's cone, which helps out the quality of the midrange sound. Lastly, a TL uses the 1/4-wavelength resonant frequency of its length in a traditional TL, and of its length and taper if it's a tapered TL, to provide bass reinforcement. The 1/4-wave resonant frequency of the length of an ML-TL or ML-TQWT is used for part of the bass reinforcement along with the size of the mass-loading port. "
Here's another explanation on the two designs that might help, Curtis:
"As the name implies, a transmission line is a long tube that expends from the back of the loudspeaker. By tapering the line, there is NO possible way in which sound can reflect back and forth and therefore standing waves and resonances common to standard speaker enclosures are eliminated. By eliminating back-wave reflections, the driver is also protected from having the back-wave re-radiate through the diaphragm, causing distortion and diaphragm breakup. The purpose of the transmission line is to eliminate the phase cancellation that would occur if the driver was in free air. Because of the length of the line, there is not enough time for air to travel through the line and cancel the front-wave. The magic of the system is what happens to the back-wave. The length of the line creates a tuned chamber much like an open ended pipe from a pipe organ. This causes a phase shift depending on the frequency and the length of the line. Through proper design, this causes the wave from the end of the TL to reinforce the front-wave at the frequencies where the front-wave begins to decrease due to increased air resistance at lower frequencies. Also, the tuned aspect of the TL strongly effects the fundamental resonance of the loudspeaker. It causes a very heavy dampening effect, which also helps to eliminate the overshoot and undershoot of the massive bass driver diaphragm. But this dampening is unlike the air spring of a sealed box and the diaphragm does not have to fight for motion. As a result, the efficiency is better than bass-reflex enclosures, the accuracy is better than acoustic suspension, and the frequency response and linearity is better than all systems."