KSA has not banned any religions. For example it is allowed not to adhere to any religion de facto and even de jure if I remember correctly.
Secondly people are free to practice their religion in private as long as they are not proselytizing another religion other than Islam inside the territory of KSA.
It might only be Makkah and Madinah that are the only holy cities of Islam inside KSA (in total millions live in those two cities) but nearly all of KSA has a historical place in Islam meaning that there are several other cities, historical regions etc. where Islam played a role. But that is less important. The most important thing is that KSA is following Islamic laws although the society is not a complete Islamic society meaning that every single law derives from Islam.
Maybe even more importantly there are no native non-Muslims in KSA. Once before the appearance of Islam KSA had significant Jewish and Christian communities while those not following Abrahamic religions (also "Semitic religions" in theory) followed ancient Semitic religions (Pagans).
For example one of the oldest churches (1700 years old) are found in current day Jubail in the Eastern Province of, KSA.
Jubail Church - Wikipedia, the free encyclopedia
All the non-Muslims are either a few hundred/thousand (at most) converts or foreigners meaning non-locals who are only staying in KSA temporarily and will move again. They mostly live in compounds and in those compounds they are free to hold masses or whatever they do.
I don't really know what Angola did fully but I don't think one can compare it with KSA for the reasons I mentioned.
That should be explanation enough I guess.