In Madara's time, war was common and i wouldn't be suprised that ninjas those days were ruthless and didnt care as much when a close friend of theirs dies. It was normal back then. They saw death the moment they were born.
Thats prolly why Itachi said in part 1 that you have to kill your CLOSEST friend...because thats prolly the only way (in those war days) that someone would get emotionally distraught,witnessing the death of a close friend wasnt enough. Madara and Izuna killed their best friends and that triggered the MS,same with Itachi who was used to people dying everyday since he was an anbu agent.
The generation of Sasuke's and Obito's were more civilized and they had a better life than the war days of Madara. Itachi is an exception because he was an anbu agent. Sasuke lived a good life until his brother murdered the whole clan. Obito also never had one of his friends killed . The MS was easier to acquire because you just had to witness a tramautic death of a loved one, instead of actually killing them. Thats how Sasuke and Obito got theirs.
Kill closest friend - Madara, Izuna, Itachi
Witness death of loved one - Sasuke, Obito