It's clear that Raspberry Pi team used Broadcom SOC, not because it was technically appropriate, but because they have strong links with Broadcom who support the project and provide cheap access to the SOC (ie. staff employed by Broadcom in senior role). The Broadcom SOC was basically an old smartphone chip I believe.
It's also clear that Raspberry Pi has built a large user community, the reason for that I believe is that RPi captured early mindshare due to access to the media, in particular the BBC, because one of the RPi team worked with the BBC on the BBC Micro project. Add to that "it's for charity"/"think of the children", and it all took off.
I also believe that a large number of sales go to commercial/industrial users who couldn't care less about the charity aspect, they just see a cheap Linux board they can stick in products.
As we have seen numerous times, the products that succeed may not be technically good, but succeed based on price and marketing.