The explanation is: It's a company decision. Nothing you or anyone else can do about it. It's between the top brass at Amazon and the music labels. Amazon music has nothing to do with Spotify. What one service has available to stream has no effect on what the other service has available.
But if the only factor for you to purchase an Echo device is for streaming music, then perhaps you're right that you should sell your Echos. After all, the sound from an Echo is no where near as good as a Hi-Fi system. And streaming music tends to lower the bitrate and produce inferior audio quality. And besides, you can stream from just about anything these days; cell phone, computer, tablet, smart TV, cable/TV boxes, streaming stick (Roku, FireTV, etc.) You don't need to have an Echo.