Basically user's time invested. They know how to use the current software "I NEED Photoshop to do my job!!!".

Even if some other software has the same or even slightly better abilities at a lower cost, the overall cost of switching is much greater. Especially if someone else is paying for the tools.

Until some software appear that make user's competition more efficient and they are forced to learn new ways to not get out competed.

Then the cycle starts all over again.