Well, 7.1.1 (System 7 Pro) was a commercial product, so that's why it was never put up by Apple for free. 7.1.1 (non-pro) was specific to the Powerbook 520/540 & Powerbook Duo 270/280, so it never saw a general release. 7.1.2 (68k) was Performa/Quadra specific and only seen with a limited number of early edition Quadra/Performa 6xx models, so it never saw a general release. 7.1.2 (PPC) was also model specific and never saw a general release.

As for 7.1 itself, I can't really explain why that one isn't available. I have seen mention of a "7.1 Update" that makes it sound like it was available for 7.0.x machines to update to a generic 7.1 release. IDK. The apple system compatibility chart seems to indicate it was compatible with a wide range of Macs from that time period:

http://support.apple.com/kb/TA47341