If you mean mainstream as in Mass population using it as a payment tool ,then I believe one thing will make it happen and that is the paying population need to be able to buy the exact same item cheaper by simply using bitcoin .

If online retailers or bick and mortar were to pass on some of the savings from not using paypal and credit cards and highlight this with a price comparison then the paying public would seek out places to obtain Bitcoin .
I truly believe until the public can see a clear reason to use it (the cheaper items ) them adoption buy consumers will struggle .

The main percentage that use bitcoin use it to store wealth just like gold and protect against fiat re-valuations by individual economies .

The current european/British legislation being implemented were depositor funds will be used to bail out banks should incentivise any depositors to think twice about holding cash in any bank .