Because the max recommended ES heap size is 32 GB. See this - Use more than 32GB for heap - #2 by spinscale. So in effect, you aren't able to fully utilise the full RAM.
So you can either set up multiple instances or you can set-up multiple VM (nodes). cost-wise it would actually work out lesser for you.
Also have a look at Managing and troubleshooting Elasticsearch memory | Elastic Blog