Because they don’t give a shit about how you license it. As long as it works and you’ll fix it if it breaks.

If you GPL the full version and provide ongoing SLA & upgrades for the software, it wont make much of a difference.

Holding back features between versions actually widens the gap between getting trial users converted to fully paid users, Mainly because the guys that try your software out want to see the whole thing, and not just a subset of features when they trial it. 

Besides, how do you think organisations that have development budgets contribute to products if they’re not GPL? More often than not, the requirement for “open source” software within a company is because they want their developers to be able to add and submit features back to the product.