There are several differences, and I'm quite sure the list that follows is not complete.

- Pressure.  
A scuba diver is subject to more than one atmosphere. Pressure increases about one atmosphere for every ten meters of water depth. Scuba divers have to worry about nitrogen narcosis, decompression sickness, and oxygen toxicity. Astronauts breath nearly 100% oxygen at reduced pressure. This means astronauts on the ISS do need to go undergo pre-breathing prior to an EVA to avoid decompression sickness, but other than this, astronauts do not need to worry about the pressure challenges faced by scuba divers.
- Cost of breathing air.  
Even trimix is dirt cheap compared to the vastly more expensive cost of the air breathed by astronauts. Even with what SpaceX has done to reduce launch costs, it is still very expensive to send anything into space, and that of course includes breathing air. 
- Exhausted air versus recycled air.  
Scuba divers simply release their exhaust breath into the water. That exhausted air still contains quite a bit of oxygen, but air (even trimix) is cheap. Because sending anything into space is very expensive, the air in a spacesuit (or a spacecraft) is recycled. The carbon dioxide created by human metabolism is captured and possibly expelled. The oxygen that remains is recycled.