A perpendicular intersection uses way less area than a roundabout. That's the basic reason.

Roundabouts have better throughput than a busy 4-way stop, but less throughput than a signaled intersection if the timing and sensing is reasonable (many signaled intersections don't have reasonable sensing). Roundabouts also have some pretty nasty worst case wait times; I'm really not looking forward to the state installing one near me on the approach to a car ferry; it won't be fun to wait for 200 cars to go by before you get a turn to go, and I expect long ferry lines to result in impatient people in the ferry line blocking the roundabout. Sometimes there's two hours between ferry loadings. Going to be some fun times.

Personally, I find it challenging to both look ahead to the right to confirm I have room to enter the roundabout, look to the left to confirm there is no traffic that I need to wait for, as well as looking far left and right to ensure there are no pedestrians crossing soon. Signaled 4-way perpendicular intersections have worse outcomes when a participant doesn't follow the signalling, but indication of right of way makes it easier to confirm at a glance if it's safe to proceed.

> A perpendicular intersection uses way less area than a roundabout.

That’s not actually true. It’s entirely possible for them to have the same footprint.