1.2.5 was awesome, but there were about four big mods that everyone wanted to use: Buildcraft, IC2, EE, RedPower 2(beta). There were actually a lot of other mods that a lot of folks were using, but modpacks were not as much a thing. So if someone knew about a particular mod that was useful and fun, they'd include it. And if something was a really good add-on or infrastructure for a mod they wanted to use, it went in the modpack. Thaumcraft really came into its own about 1.2.5, as did Mystcraft. Logistics Pipes was THE premiere automated way to do logistics, while RP2 did the grunt-work of mobile quarries and making a sorting system functional.
Direwolf20 was really getting to be a big name in Let's Plays, and because of his videos, people started to promote the hell out of mods and learn how they worked. But modpacks were difficult and you had to download and install everything piece by piece. A large modpack had maybe 25 - 35 mods in it. The Technic Pack launcher was a thing by then; the problem with it was an issue of permissions, as the Technic folks didn't want to ask permission for IP which was freely downloadable, even when asking for permission was the ONLY thing the modder wanted them to do. So this led a lot of modders and modpack makers to avoid the Technic launcher.
Then came the 1.3 update, and some of the big things it changed were the graphics rendering system, the way the save files and world-map data were structured, and the code-scrambling obfuscation. Forge took a while to update because of the complexity. RP2 vanished because it was just too much work. Eloraam was going to skip 1.3 and go to 1.4 when it came out, just because of the time it was taking to figure out the new rendering and get Forge running. With a major hole in the general mod ecology, a lot of people went looking for alternatives. By 1.4, RP2 was forgotten and Buildcraft was huge.. leading people to look for alternatives. IIRC, in 1.5, Thermal Expansion and its itemducts came on the scene, and RP2 was sadly left behind for what we thought was good. Resurrection projects had recently manifested to get the best parts of RP2 back in the game, starting with red alloy wire and logic gates.
1.3 caused a lot of mods to stop updating as modders took forever to catch up. In 1.4, many did catch up, while in 1.5 a lot of new modders were now participating. Also with 1.5, the FTB modpack turned into the FTB launcher (which had the original 1.2.5 modpack and FTB map, but focused more on modpacks in general). On top of that, the Technic folks, having figured out that asking permission meant a LOT more folks were willing to have their mods in modpacks with a lot fewer complaints and legal threats, changed policy to suit, and now ask permission.