TL;DR at the end.
The midbrackets (29s, 39s, 49s, and 59s) each brought features that kept players coming back for years, well after the XP-off split in 3.2.
29s were the first bracket that, with some work (and really long quest chains) granted access to TBC enchantments, which required lvl 35. Back then, enchantment requirements were more complicated -- they prioritized required level over ilvl, so the key for 29s was to find gear that had no required level because it came from a quest chain, but still with an ilvl of 35+. A lot of gear for 29s was spec'd for characters well into their 30s, so you could get some great stuff with powerful enchantments.
39s found access to a lot of blue gear from Uldaman, and a couple of great quest chains. It was also the first bracket to allow primary professions to reach 300, opening up serious engineering items including the long-famed Green Lens. The Pulsating Crystalline Shard was so strong, it was BiS for casters in the 49 bracket as well as 39s, one of three items in the game at the time that had a BiS item shared between brackets. Talent choices started to make a significant impact. 39s developed a culture of high expectations, and their PuGs during Wrath were the best any twink bracket ever saw. I respect that 19s could have some great PuG games, but 39s demonstrated what team coordination and communication could do, day in and day out.
49s had the highest gearing challenge in the game. Near impossible At'Alai drops out of the original (much, much larger) Sunken Temple and the rings and amulets from the relic coffers within Blackrock Depths meant that one BiS piece could take as long as an entire BiS 29. The only grind I've ever heard worse than 49s was the Ring of Precision for 20s and satchel stam shoulders for 39s. We're talking thousands of runs. 49s had the highest burst in the game, so players really had to learn field positioning. Not many had the patience and tenacity to play 49s, as the run-and-gun playstyle proved too frustrating for most. One mistake, and you got globaled.
59s allowed for main professions to reach 375 (making engineering a requirement for all 59s), plus access to the Zul'Gurub raid. Combine that with the first bracket to obtain TBC gear, and you saw tremendously strong 59s who could reach deep enough into their talent trees to support some wildly disparate playstyles. The bracket obtained a reputation at the beginning of Wrath for its DK-infestation, but everyone who played after 3.2 knew better -- only a couple of players could make DKs compete with the rest of the classes at 59. 59s were the midbracket that most quickly organized after the XP-off patch threw twinking into chaos, but interestingly, they never recovered after games dried up for nearly all twinks at the end of Wrath.
TL;DR/summary: the midbrackets bring a longstanding reputation as the best PuG organizers in the history of WoW twinking. Other brackets talk about "the community" and balancing games -- that often stemmed from the years of work players put into the midbrackets. Because making games happen in the midbrackets was tougher than the lower and upper brackets, players learned the kind of leadership and organizational qualities needed to retain and develop its playerbase. I'm not taking away anything from the well-deserved accomplishments of other brackets -- those also bring a great depth of history. To answer the OP's question, then, when you ask why people played the midbrackets, the primary reason is always "the community". Gear brought them there, but the midbracket communities found ways to keep coming back and play together time after time.
