Smoking the bowl down to the botton can be based on three things. One, if you are smoking an aromatic they smoke whetter than and English blend and therefore it is hard to smoke to the botton because of the collection of moisture. Two, it all depends how you smoke, as mentioned, a harder puffer rather than slow mini draws usually collects moisture at the bottom of bowl because of your saliva and the burning qualities of tobacco which when burned creates a little moisture. Finally, tobacco packing a tamping is important since generally the tighter the tobbaco is loaded the more moisture will be created at the bowl's bottom. Try packing you tobacco looser which should help.
The most important thing, however, is don't sweat the issue of moisture at the bottom since the very premise of pipe smoking is ENJOYMENT and there is nothing that is going to happen to your pipes if you don't smoke all the way down to the bottom of the bowl.