Because that stuff is managed by the debian-provided rtlsdr packages on jessie and newer. wheezy is only special because there’s no prepackaged librtlsdr available.
I see.
Still it was convenient for me to blacklist two additional drivers on Raspbian Stretch Lite.
I guess if detaching the kernel driver is handled properly there is no real need for blacklisting.
And just that is done by -DDETACH_KERNEL_DRIVER=ON i suppose.
Thank you for explaining 
Still curious why librtlsdr would blacklist two drivers, but not others.
Or do these two files belong to another package?
/etc/modprobe.d/blacklist-rtl8192cu.conf
/etc/modprobe.d/blacklist-rtl8xxxu.conf