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the more passes, the more instruction slots you use which leads into a decreasing framerate like you mentioned. This is also partly caused because when using one technique and several passes, the technique encapsulates the effect state that determines a rendering style. A technique is made up of one or more passes. An effect file that uses one technique and multiple passes like yours, has result that one pass depends on the result of the previous pass so you get a fps decrease.
This explanation leaves something to be desired..
The basic reason multipass shaders like this are slow (aside from the mulitple draw calls and polygon transforms) is the excessive overdraw.
Think about it, each pixel in the effect has to be redrawn at least 18 times, often more.
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