Why would many people want to use only Python in designing any Machine Learning project. Machine learning, in layman terms, is to use the data to make a machine make intelligent decision. For example – You can build a spam detection algorithm where the rules can be learned from the data or arranging your email based on tags you had assigned by learning on email history and so on.

Machine learning is nothing but to recognise patterns in your data. An important task of a Machine learning engineer is to extract, process, define, clean, arrange and then understand the data to develop intelligent algorithms.

Machine learning engineers or Data Scientists recommend Python, because it’s easy to understand.

Sometimes the concepts of Linear Algebra, Calculus are so complex, that they take the maximum amount of effort. A quick implementation in Python helps a ML engineer to validate an idea.

If we focus on the overall task which is needed to validate and test the models — as far as it satisfy the aim of the problem, any language/tool/framework can be used. 
But the main reason for using Python would be its readability, versatility and easiness.