Complex sentence: An obvious reason is that while the thread is blocked, it cannot accomplish anything: if the blocked thread had been performing a high-priority or real-time task, it would be highly undesirable to halt its progress.
Split into simpler sentences: An obvious reason is that while the thread is blocked, it cannot accomplish anything. If the blocked thread had been performing a high-priority or real-time task, it would be highly undesirable to halt its progress.