Sometime during the 1500's the magisterial reformers abandoned the idea of Sitzerrecht - that all believers have the right and duty to test teachers and determine the meaning of Scripture together - and embraced the principle that only a "technically qualified theological expert" could properly interpret Scripture for a gathered group of believers.
The idea that only a qualified expert can exegete and explain Scripture today is embraced by most congregations - even if it is not voiced as a hermeneutical principle by those same congregations. For this reason, the "sermon" and the "pulpit" are placed in a superior position to any other type of communication between believers. Since the sermon is now in a superior position - for many a sacrosanct or even sacramental position - Paul's instructions in 1 Cor. 14:26ff. concerning interrupting a speaker are considered to not apply to sermons.
Fascinating stuff. Too bad the magisterial reformers (a certain Zwingli included) didn't stick with the scripture, but instead substituted their own version of the Mass (he says, ducking!). Read the whole post for more background