OTF metering question

[Steve Jolly]

I'm sure there's a good reason for this, but it's not obvious to me. 
Every modern Pentax film body (from the SF series onwards, plus the 
Super-A) features TTL flash metering, and hence has a second OTF light 
meter.  As far as I know though, only the LX offers OTF metering for 
available light photography.  Anyone know what the technical limitation 
is?

(This post inspired by today's enablement with a Super-A, courtesy of John 
W...)

S