Adblockers on iOS require the 64 bit A7 SoC which was released with the 5S. I'm not sure if there's a valid reason why they couldn't make it work on 32 bit too, whether it was just extra effort or whether they'd have had to make technical concessions, but the way they implemented ad blocking within Safari is different to other solutions (e.g. the side-loaded ad blockers that 05 mentioned) so it's hard to be sure.

For more information on WebKit content blockers: https://webkit.org/blog/3476/content-blockers-first-look/

Update: performance is the reason that it's 64 bit only https://twitter.com/awfulben/status/638526406805229568