Optical turntables have major issues with dust and dirt. The records need to be extensively cleaned prior to being played [1], even for unplayed "virgin" vinyl. 78rpm (shellac) records are older than vinyl records, and tend to have more dust / dirt, so would require a lot of cleaning prior to archiving.

Although the more expensive of the optical turntables (the LT-master) has 78rpm capabilities, such capabilities are optional even on this high-end version.

I think it's basically down to costs - the costs of cleaning each record (I know they're cleaned already, but the process may need to be more involved to remove all dust), along with the tens-of-thousands for the player, probably don't make this economical on the scale the Internet Archive are looking for.

[1] https://en.wikipedia.org/wiki/Laser_turntable#Performance