Because you then have to check and store 10 minutes worth of packets instead of only one number and the query lifetime worth of packets.
And because a slowly changing "connection quality" is a neat GUI feature.
To make it visually appealing it would be possible to just substract the minimum connection quality and multiply the result with (1/(1 - min. quality)), so users see "OK, it went negative, so the node was disconnected". Could even be done with a simply 0%-100% "progressbar" which goes red when it closes in on 0 and turns dark read when the node gets disconnected (<=0).