Why is RSA encryption version 1.0 not specified in an RFC?
RSA is and was specified by the PKCS#1 specifications of RSA laboratories. PKCS are the "Public Key Cryptography Standards" by RSA Laboratories, now part of EMC2. The RSA PKCS#1 v1.5 is the lowest publicly released version of RSA by RSA labs that can currently be downloaded.

Version 1.0 to 1.4 are working drafts as specified in the PKCS#1 documents themselves and at least the inital PKCS#1 RFC 2313 as well:


Versions 1.0–1.3 were distributed to participants in RSA Data Security, Inc.'s Public-Key Cryptography Standards meetings in February and March 1991.
Version 1.4 was part of the June 3, 1991 initial public release of PKCS. Version 1.4 was published as NIST/OSI Implementors' Workshop document SEC-SIG-91-18.


Although the RSA standards have been defined by RSA Laboratories they are likely to have been influenced by the participants to the meetings and workshops.

The RFC's are simply a reiteration of the PKCS#1 standards that have undergone public scrutiny. They are not in control by a single private party. So the standard may be identical, but it is at least open. In general the RFC's just keep to the PKCS#1 specifications, but at least in principle follow up documents may deviate from the specifications by RSA.