Hi,
Why AAC not suitable for broadcasting purpose?

It is! HE-AAC is used for digital radio in the DAB+ system, which I'm happy to say started broadcasting in Australia this year. I believe the older DAB system used MPEG1 Layer II.

Why AAC-LC is not suitable for broadcasting?

I believe in Australia, AAC-LC is being used for music stations at a 128 Kbps bitrate, while talk back, news and sports stations are using 64 Kbps HE-AAC.

HE-AAC uses Sub Band Replication to improve audio quality at low bitrates. The sound isn't accurate to the source, but it essentially is designed to not sound annoying. This means you can have stereo streams at low bitrates that using the old DAB MP2 system had to be encoded as mono to avoid harsh compression artefacts.

How to avoid the delay in broadcasting while using AAC-HE instead of AAC-LC.(Since most of the tools are mandatory with AAC-HE and More data needs to transmit than AAC-LC)