They haven't done this for vulnerabilities in either 3.1.3 or 4.2.1, the other two EOL versions. I guess there probably aren't enough users of these to make them attractive to exploits, but the number of users stuck on iOS5 will likely stay in the 8 digits for a while. Whereas many people have been conditioned to buy a new phone every 24 months, I doubt this applies to iPads and iPod Touches as well. I don't know if that kind of number makes them a sufficiently attractive target for criminals.