There are many reasons behind this, but I'll try to summarize the no-brainer ones:
- PHP is free software and Oracle is strictly proprietary. Oracle will do little to help the free community around the PHP codebase. No matter what you do, you'll have to cross this war of religions somewhere along the ride.
- PECL OCI8 code is about 8 years old, and a handful of people with brave souls are still struggling with crashers, leaks and incorrect behavior. I have full respect in what they are doing, I know it needs to be done, but they desperately need a lot more help.
- Oracle database is multiple decades old as well, so it surely has a trunk full of old dirt, and I guess that's confusing developers integrating it with third-party software, like PHP.
- Oracle does not provide enough documentation and examples available for public use of C libraries of OCI, this alone limits the contribution PECL-OCI8 module might have received if at least the OCI protocol specs were publicly available, let alone the sources of the client library.
- Oracle database is usually under heavy load, so is PHP when used together. Experienced developers know that race conditions, locking issues and even memory leaks are a lot more easier to observe under heavy system loads. You'll most certainly experience odd behavior of PECL-OCI8 code if your database or website is going to work under heavy load.
- Oracle is expensive, and in my opinion about 60% of the user base doesn't really need it. You don't have to finance Larry's big toys.
If you ask for my advice: don't. Go get yourself a breath of fresh air.