Because many airports are made by users, and usually small airports require less work so they are made easily, using Laminar official and approved libraries; they are provided with the sim (and updated or added at each sim update). Others are available free through the .org repository, many of them using additional libraries (so they cannot be provided directly by Laminar with x-plane). You have to download and install them manually, together with the libraries required. Manila is one of this.
You can also find some very nice payware at the .org store or at some other online store.