First of all, slavs never had khans, the supreme title was "kniaz" which roughly translates as "prince". The Kniaz'es rule could be best compared as a limited monarchy, as opposed to the Khan of the Eastern people who was an absolute monarch. There is an easy explanation for that. Most slavs were "landed" i.e. they lived of the land in permanent settlements. Agriculture was the most important way of making a living. Every family clan owned land and they owed nothing to no one. They were not warlike and would accept the general rule of a supreme power as long as they were left alone to tend their crops. Many settled in the Balkans with the permission of Byzantium and got along with the Empire fine for decades. The Elders of each clan formed the Veche which was the parliament (this is 5th, 6th century and on). The Kniaz presided over the Veche but did pay attention to what the Elders had to say. In war the Kniaz was the supreme ruler and commander. This type of government was called "Military Democracy" (that's a free translation, so don't stress on the name, but should get the idea). Democracy was not very suitable for a country's formation or its protection. In order to survive in the Dark and Middle Ages there had to be a strong hand to rule. And that's where the foreign element comes along. In the creation of Bulgaria it was the Bulgars (nowadays it is widely accepted that they were of alano-sarmatian origin, as opposed to turkic). The Bulgars had a monotheistic religion - one god - Tangra. The slavs worshiped many - Perun, Volos, Lada etc. The Khan was the absolute monarch who enherited his powers through his father with the approval of Tangra. The slavs elected their Kniaz among the Elders. The Bulgars gave that strong foundation for governship without which they would have never survived in the steppes to the north of the Black Sea and without which the newly created state would never have survived among strong neighbors as the Byzantine Empire, the Frankish Empire and the Avar Khaganate. It is the same with the Eastern slavs and the Varangians or the Rus. They brought that statemanship skill and united the loose alliance of the slavs. Eventually they melted in the vast sea of slavs, but not without leaving the Rurik dynasty that ruled Kiev until the Mongols.