Yemenis in Yemen, have been bombed by a much wealthier country than it. Why? I presume for the same reasons that many other Arab nations are at war with one another with so many intervening nations. The reasons always seem to be: money, religion clashes, sectarian differences, old feuds, colonization, suffering economies, diminishing resources, exploit, rivalries etc.
How nice it would be, if we could get a specific statement from the Saudi Kingdom, as to why they bombed Yemenis.

Update 4/25/18: Well, the answer I asked seems to have been answered. I refer you to the following article (which has been very helpful to my understanding): Houthi insurgency in Yemen – Wikipedia