The "why" isn't the problem. Sound changes happen all the time, and they're based on increasing dominance of dialects due to a number of possible historical factors. The loss of dental fricatives in German (except for voiceless s, obviously) occurred somewhere between the 6th and 8th centuries in what we can observe as the written Old High German language. This was the time of Frankish and Catholic dominance, and the increasing impact of Latin in Germanic territories. Since Latin does not have these dental fricatives, this got lost in the occurring sound change. Thus, the th-stopping occurs in German, Dutch and Friesian today. If you look at the Germanic languages that retained the dental fricatives in question, you can see that it's the Scandinavian languages and English - both outside of the Frankish sphere and thus isolated from the Latin impact in the time of the sound change. By the time Catholicism did arise there and Latin entered every-day life there, the English and Scandinavian languages already got so fixated that it had little impact on them. It's interesting to note however, that Swedish did lose those sounds, but honestly, I don't know when that happened.
