At first, I assumed it was due to the rise of payment processing services like BitPay and Coinbase which shield merchants against the risk of BTC/USD price volatility by automatically converting any coins received into fiat since the use of such payment processors is something that these companies all have in common. However, BitPay was founded in 2011 and Coinbase was founded in 2012 so that doesn't explain it.
The only other explanation I could think of is that Overstock's announcement of Bitcoin integration provided such a boost to sales that it served as an example to the market and so other companies also followed suit. Their marketing departments saw how services such as BitPay could allow them to accept BTC with no risk to them since they would still be getting the same old fiat in the end. Meanwhile, they would benefit from the positive publicity as well as increased sales from Bitcoiners.
What do you think? Do you think this the only reason or are there other factors at play? And now that accepting Bitcoin is no longer viewed as "groundbreaking" as it used to be, do you think the trend of ever-increasing merchant adoption will continue into 2015 and the coming years or have we reached a plateau?
You are missing something that is needed here. That's is time (even though it is non existent, but that's a different matter). Nobody could expect a big company to adopt Bitcoin, half a year or even a year after Bitpay. Even though they're protecting the marchants, that's not enough. You don't just go around the internet and adopt any payment option that you see. Even today, there is a lot of misunderstanding when it comes to the use of Bitcoin.
Overstock was the first "big" company that started accepting Bitcoin. One could have expected that there would be a boost in sale which stagnated afterwards. This helped out in a way. Everyone who doesn't accept it kind of stays behind and missed a possible market.
I definitely think that adoption will continue, maybe even on a quicker pace. I don't see it reaching a plateau because there are so many merchants in the world that could start adopting it.