Why does PayPal have such enormous power? What's stopping other initiatives from getting enough traction?
They have a huge amount of inertia because the average guy of the street has heard of them and can trust them to some degree.
Trying to get people to sign up with a new, unknown payment processor during checkout will have a strong negative effect on your conversion rates.
"...there is also growing evidence that increasing numbers of people today are unwilling to engage in social or economic contracts based on trust. Globalization has arguably contributed to a loss of social connectedness in many developed and undeveloped nations by disrupting traditional employment patterns, and people who have seen their social networks disrupted may be reluctant to trust others.
Meanwhile the traditional family has been under assault by other social changes, and young people may find it difficult to even learn to trust. For many people, relationships based on faith or confidence may feel more comfortable than those based on trust. Source (2005): http://www.brandchannel.com/features_effect.asp?pf_id=263
Not having Checkout/Wallet as a popular payment system has also been (and still is) a major obstacle in getting people to buy apps and content from the Play Store. If Checkout accounts were as popular as iTunes or Amazon accounts at least, things would've gone a lot more smoothly, and I'm sure more merchants would've preferred it over Paypal, too.