It has to do with USB standards. By USB standard, one side is host (provide power) and one side is target (receive power). Also part of standard is that the connectors are designed so you can't mistakenly plug in cable incorrectly. As such, the host side is always terminated with standard USB A connector. The target side can not use the USB A connector as it is reserved for host side, but can use basically all the other USB connector types (B, mini, micro, etc) as target. As such you will never see two host connectors (ie.A to A) or with two target connectors (all the other connectors to any other connector but A).
With the OTG USB standard extension, it allowed all sorts of USB connectors to connect to each other. The OTG standard allows this as the OTG standard requires some type of negotiation between the devices as to which device is host and which is target. Since OTG is so new in its adoption, there are very few cables made for it yet as OTG devices are only recently making it to market.