* If you are upper level management at Blue Chip company "Initech", the last thing you want to be dealing with is a smaller B2B provider, with a hipster name like "Data Badger". Being smaller means more risk (not necessarily but thats the culture). You want the big safe boring companies.
* Big corporates are still wined and dined by Oracle's sales teams etc. Yes you might have to do your mandatory HR training about bribes and corporate gifts, but this stuff still happens under the table. Its business, it might not be ethical, but its business.
* Cronyism etc. management tends to circulate around companies closely connected with each other. A Microsoft manager might float to Vodafone or JP Morgan. When they get there, they are more likely to throw any of their old MS colleagues a bone when it comes to service contracts etc. This is why networking is so important in LinkedIn for corporate careerism.
* Did I mention sales already, yes, but these big companies have large sales and marketing budgets. Those buzzwords are expensive you know!
* Those sales and marketers target managers and people with a high influence / low technical understand co-efficient. Don't get me wrong there are smart managers out there that know their stuff however they are generally a rareity. So they tend to drink the kool-aid from the big tech corporates e.g. "Hey Bob, I think Microsoft's Foobar Cloud Hypervisor, can streamline our data acquisition and increase synergy in the business".
* When you take your first hit of Oracle's software or services, you are effectively tied into service contracts which make it painful to leave but easy to extend. You might spend £200million for a 5 year contract, then three years in you see a new technology offering for an extra £4million etc. That's relatively pennies to what you've already sunk into the service contract.
* Marketing and branding which you picked up on in your post. There is a reason you see IBM or Oracle advertised a lot in the Financial Times, Economist, New Scientist etc.
Hope this explains why Oracle et al. are dominent in the industry.