Instead of why? how about using tail -f which is meant to solve this problem.

Code:

nc -ul -p 56045 -q -1 | tail -f | awk -F: 'NF > 1 {print strftime("%d-%m-%Y %H:%M:%S"),$0;fflush();  }' > awk.txt 2>&1 &

What you need to consider, if you do not like tail, is to develop some kind of daemon - usually written in C. You have an attached child process that leaves the parent there forever. If I understand what you have there.

PS: fflush() on every line forces disk I/O instead of buffered I/O. Since nc can produce an endless stream of data this change might help.