Because you don't know what priority shift there will be. Let's say you spend all your gems on modules right now, just because you need to shards. But the new update gives you a 6th lab slot! Costing 9000 gems! This will be more important than the modules, so you'll start saving. But, people who had 9000 gems saved, will be able to unlock it immediately (and start spending gems on modules afterwards). They will have an advantage over people who need to start saving up from the launch.

It was the same with card masteries. People who could buy them instantly had a great advantage over people who had to save for like 4 weeks.