Android phones are cheaper than iPhones. That's all it is.
Not that there aren't other important factors for Android's success, as even relatively expensive Android phones are quite competitive with iPhones in market-share. But even if you assume it is just price, that is an argument in favor of the multi-vendor strategy, not against.
Glass was purposely priced out of the range of consumers. It was meant as an experimental platform only.
Still, it would have been priced higher than relative high end Android phones, have worse specs than mid-level phones and be perhaps better only in consistency and software-hardware integration (did I just accidentally describe the Nexus line?). It would not have beaten Apple at its own game and without a larger market-share and more friendly developer terms, it would not have had enough apps to achieve critical mass (and now I seem to be describing the Windows Phone...).
It's amazing to see Samsung in so much trouble now that Apple has released the iPhone 6/6+. They have lost a staggering amount of profit in only a few months.
Samsung may have lost some market share and profits when the iPohne 6 was launched, but it really started bleeding when it stopped any pretense at innovation, and it just kept iterating on the ugly design of Galaxy S3 and the bloatmess of Touchwizz. Galaxy S5 was considered a failure. Because of that Samsung intends to "rebuilt everything from scratch" with Galaxy S6. It's not because of the iPhone 6.