Question

why thermoplastics become soft on heating and thermosetting plastics do not

Answer 1

Thermoplastics are made up of weak linear arrangements and are not very hard on heating the weak linear arrangements breaks easily and thus the plastic melts. On the other hand, thermosetting  plastics have a very strong cross linked arrangements which does not break on heating and thus the plastic does not melt.

Answer 2

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Thermoplastics can be softened by heat but thermosetting plastics cannot be softened by heat. This is due to the difference in their structure. ... Due to this, on heating, the individual polymer chains can slide over one another and thermoplastic material becomes soft and ultimately melts.