~20M transaction per month are an arbitrary limit. The economic majority recently decided against increasing that limit, but this doesn't mean it's not going to get increased at some later point.
Of course there are technical reasons for that limit. But whatever the correct size of the limit is - the current limit is almost certainly too low. The Ethereum blockchain is able to handle a significantly higher amount of transactions without implementing all of the performance optimizations that Bitcoin has (such as compact blocks).
