It has a Mediatek soc, custom roms for these chips are scarce. If you look at the supported devices on the Lineage wiki, you’ll see only 2 out of 550 devices have a Mediatek soc[0], most of them are Qualcomm.

And iirc from the xda forums, even for Xiaomi phones with a Qualcomm soc it isn’t certain anyone will try to make a custom rom. Xiaomi just releases too many devices to have support for all of them.

[0] https://wiki.lineageos.org/devices/