These days, PostmarketOS is trying to support those older devices, and in a way that actually takes out Android! ...out of the device I mean, haha. But IIRC, most devices that CM used to run on, only had 512MB of RAM-- and that's honestly not enough to even browse the web on nowadays. (Many of them are also on earlier flavors of ARM that would be a bit harder to support natively, with e.g. Alpine having dropped support for anything earlier than ARM7.)