Probably because plastic guards don't have that little lip that metal ones do so water has a free run to the edge. You might be able to improvise something to catch the water at the edge and keep it in the guard, or just get some GBs.
Or just get the proper size, usually 10-12mm wider than tire.
@withered_preacher started
London Fixed Gear and Single-Speed is a community of fixed gear and single-speed cyclists.
Probably because plastic guards don't have that little lip that metal ones do so water has a free run to the edge. You might be able to improvise something to catch the water at the edge and keep it in the guard, or just get some GBs.