Trophy and similar weapons work by sending a cloud shard of metal shards in the direction of the target. Given that a presidential convoy will often be in the presence of large amounts of civilians, using active defense system may mean shredding a couple dozen voters (possibly on live television). I can't imagine many politicians being very happy about that.

I think DIY weapons would be the more dangerous threat btw. A sharpened stick on one of those racing drones could rest on top of (say) a small shack until the target comes by, then strike extremely quickly.