Why don't red blood cells pop in the bloodstream?

Red blood cells do not pop in the bloodstream because of the osmosis, a specialized case of diffusion. Osmosis is the movement of water through a selectively permeable membrane from a region of its higher concentration to a region of its lower concentration.

Because the blood osmotic pressure is isotonic

Osmosis happens only when there are differences in the concentration of the ion. If the concentration is same, called isotonic, the osmotic pressure will be same and no water will be moved.

This is why resuscitation fluid mostly used isotonic solution (0.9%NaCl), to prevent osmosis to the cells.