Comparisons and advocacy

Both and LMS and a MOOC platform are tools for education. If you start by discussing scalability, you are missing the point.

Think instead about exactly how the roles of teacher and student are played out in your context, then you see what interactions the tools have to support.

"So if i take a Open Source LMS and host it on Cloud, The problem will be solved."

It depends on what cloud service you buy and how much you are willing to pay for that service. The cloud is just sombody elses computer, it doesn't make performance issues just go away.

If you buy a service from a major supplier and decide to limit the resources (e.g. for financial reasons), then you may not get the performance you would like. If you don't limit the resources you might find that offering the service is expensive.

"So why everyone rush for the development of MOOC Platform."

I don't understand your question, what do you mean by MOOC platform? MOOC is really an educational concept. You can deliver through any set of software that fits your purposes.  Big venture capital funded operations have developed their own special software and I think they are going to lose large amounts of money on the operation. 

Other people have taken existing VLE software and used it for many MOOC concepts such as teaching people about Moodle etc. What do you want for your learners?