Re: Slightly off-topic question
IMHO - There are a few reasons I can think of, in no particular order.
Legacy: Once you have a DB platform of a significant size and you didn't set it to ANSI SQL from day one then the transition to a difference platform is substantial and fraught with risks and potential downtime. A high transaction database in a 24/7 operation with need for existing and current data to be maintained is a scary prospect. In reality there were few scalable, fast databases around - nowadays many organisation could choose a range of database options for a new product and there would be many better options for most than Oracle. There is also DBA experience. An Oracle DBA is unlikely to support or be very effective if there is a move to another platform. Generally they will move on to another Oracle DBA role rather than retrain.
Product integration: Oracle are pretty good at getting their database used as the backend for a number of products in certain sectors. Therefore a company goes out to buy a product and chooses it based upon features however the underlying database is Oracle which wasn't necessarily one of the considerations at the time. As long as you are bound to that product then you are also bound to the underlying supported database.
Great deals/C suite schmoozing: Oracle, in my experience, are better at 'reaching out' to executives and offering 'great' solutions at a 'great ' price that will solve all their problems and needs. The reluctant IT bods are overidden as Oracle have already predicted this and put the FUD about the ability of the IT team to the C Suite at the initial meeting. The conversation the Cer then has with Oracle goes along the lines of 'My IT Manager tells me that you will make me slaughter my first born when it comes time for renewal?', Oracle replies 'Of course we won't, James. Remember everything we discussed. Would we be used by so many companies if that was true. You know you were having doubts about your IT manager, well I think you can see now what I was saying about their unwillingness to innovate and realise the true potential of new products. How about we set up a small cloud agreement FOC and transfer some of your workload to it to show you how good it is - Free, yeah, no risk?'
Evaluation: Some people may choose Oracle DB because they have done extensive independent testing of the available systems using a wide combination of hardware and client optimisation and for their workload it provides significant performance gains that are crucial to the running of their business. They've found that by going with an alternative system would cause serious competitive disadvantage. These organisations are prepared to pay whatever it requires and while an alternative product may have better performance in some scenarios, this scenario happens to work best with a certain configuration for an Oracle DB. After a few years see the Legacy reason above. These companies are quite rare.