I work with a lot of younger musicians in a church and community band context, so this answer is based on my experience. Here's some contributing factors:
Popular Music. Don't underestimate the effect that mainstream pop music has on the instruments that people want to learn. This contributes to the popularity of guitar, bass, drums and keyboard. Synths are (currently) heavily used, so many people want to know how to program them. This increasingly includes your 'bedroom producer', who only uses a MIDI keyboard and a computer. People want to play the music that is part of their culture, so they are more likely to learn those instruments. When was the last time you heard a bassoon solo in a pop song? Probably the Beatles. It's always the Beatles...
Expense. Some instruments are cheap. Others are not. Have you seen how much an oboe reed costs? You can get a cheap (and nasty) guitar or keyboard for a hundred dollars in ALDI (insert another supermarket as appropriate). Even within wind instruments this can be a factor. Trumpets are cheaper than saxes, at an equivalent quality level. As you raised, percussion (that isn't drums) is often quite pricey.
Ease of playing. Some instruments take more work than others. This is multi-level; it's easier to get a decent sound out of a keyboard than a saxophone, and it's easier to get a sound of a saxophone than an oboe. French horns are harder to play than trumpets. Our top-level marching/concert band has seven trumpet players and one horn player, and there are far more trumpets in our junior bands than trombones.
Specialised instruments. Some of the instruments you mention are second instruments. You don't just learn piccolo; you learn flute, then piccolo. You are unlikely to start on Sousaphone; you'd probably start on tuba at the least, or maybe baritone/euphonium/trombone. As a result, there are fewer players. There are often also fewer parts written for the instrument, which exacerbates the 'problem'.
