I imagine it's a combination of all of them. Phinny was massively popular, but there was only a few big boxers (6) and VERY few 20 boxers. Phinny also was fortunate and got the event server that replaced a real server, so it didn't have the massive drop off 1 year in. The following server was also... Agnarr, which helped it keep people even longer lol.
Mangler also kept a lot of its core like Phinny. The difference would be the amount of large boxers. Mangler, during planes of power, had at least 250 characters that were played by like 4 people. That's a huge boost in the population by itself. 250 is a conservative number. That's just raiding toons. There were 10 accounts just being used as bankers for them. During free to play on Mangler, there were 80 lvl 1s in sirens grotto that were keeping 8 picks open and then 6 lvl 65s in each of those picks farming sirens. Along with 2-3 picks of plane of water and 2-3 picks of sol ro tower and 2-3 picks of tactics.
Imagine casually warping 80 lvl 1s on a truebox server into Sirens Grotto, so you can farm 8 picks of SG with your 8 groups of lvl 65s.
RIP Truebox