That is a tough question. If you are familiar with the recent events in regards to the block size limit being raised and Bitcoin XT you probably know why.
Bitcoin has too many people that are closed-minded. The problem is that Bitcoin can't handle the needed amount of transactions for it to become mainstream.
20 MB blocks would be useless if we got anywhere near mainstream.

I guess it is going to take a combination of block size and other technologies (e.g. Side chains; Lightning network) before the network is ready.
I think that it is going to take us a few more years. I'm not going to mention a specific number as I will probably be wrong.