My main reason for saying this is because bugs with exoskeletons survive much better than bugs with endoskeletons or (surprise surprise) no skeletons. If your skeleton is outside your body, it protects everything inside you. Endoskeletons merely help hold you together.
This process would be the one truly responsible for the "evolution" of mitochondria and chloroplasts as we know them from their prokaryotic ancestors. The cell process that would have initiated endosymbiosis is endocytosis; the process of taking things inside the cell