Because regicide is one of the worst offenses in westeros, especially considering Jaime was a member of his kingsguard and technically on his side. So betrayal, regicide, breaking of his oaths under the seven. Sounds pretty bad to me. The only reason he wasn't executed immediately after is because the side that won benefitted from it.
Also keep in mind that not everyone in westeros supported Robert's rebellion. The Tyrells and Martells were allied with the Targaryans and the common people weren't really effected by Aerys' madness and didn't care too much for a war which Robert started and was responsible for, as far as they were concerned.