There are many reasons why but a lot of it is because of the below:
- Pre-existing communities in certain brackets
- Level locked characters put into separate battleground queues
- Level locking was only released in TBC I believe
- level bracket changes
1.
The 70 community is a great one to look at. Back before WoD you could gain rating in the arena brackets. I can't remember the full conditions but I believe that to be in a team and earn rating you all had to be level 70. This is a clear reason why people stopped at 70 and decked out their twinks.
Since WoD you can't gain rating but those characters that were played for years accumulating heaps of bis gear were reluctant to level up to 79, the top level for the battleground bracket. Many others were in the same position and kept their chars at level 70 enjoying themselves dueling, queuing for 2v2 w/o rating and queuing for bgs.
This is a very similar story for the rest of the brackets.
2.
There was a time from (may be wrong) Cata until late Legion where if you locked your xp bar you would be thrown into a different BG queue. These queue's were for Locked vs Locked only. All the levellers were put into a different BG queue as to not get stomped by over geared twinks.
This would often mean that groups queued up into each other in the 60, 70 and 80 brackets. A lot of this also comes down to sportsmanship too. Teams wouldn't take on a 74 or a 64 because it was just unfair. They enjoyed the level playing field that each had and it grew a good sense of competition between factions and battlegroups.
3.
Level locking was released in TBC. This doesn't have much to do with anything apart from the fact that 19 vanilla twinks were the best to go for as if you accidentally levelled then you wouldn't have to invest so much time into levelling a new one.
It's a big reason that 19 is so popular because it gives you the least levelling to gearing time spent ratio.
4.
I believe from Cata to start of legion the brackets were 10-14, 15-19, 20-24, 25-29 ect. This made the difference between a 60 and a 64 much smaller than the current 60 to 69. The 64's for example were few and so the 60 community didn't really concern itself with the drama much. This was the same for the 70-74 bracket and the 80-84 bracket. They weren't too different in other words.
Other points to note are style. It feels good to own a 60, 70 or 80 twink with the expansions best in slot raid gear. There's nostalgia that comes with it too. Grandfathered pieces that suit the character and it's level instead of levelling up 9 more levels with an item designed for level 70's.
Sportsmanship is another part. I understand that Twinking is all about maxing the character out for its bracket but others also enjoy the fair (**Sul'thrazLOLRamstiens**) competition between each other.
It's a game at the end of the day so roll whatever you fancy. I have characters at 60 because there's a large community there and I can log on and queue a BG with 4 other twinks whenever I like. I also have an 89 because I really enjoy the bracket & the fact there's 2 expansions between 80 and 89 make the decision harder.