Because

Not entirely sure what you're trying to say, but if I understood you correctly, then the SR25 is originally created for single shot - in real life, that is. The Airsoft models are made for the same purpose as every other airsoft rifle - puking plastic BB's. When given a higher muzzle velosity, it can be hard on the mech box, therefore most owners choose to use SA fire only. I hope this answered your question? If not, feel free to PM me!

AFAIK, the SR25 uses 7.62 caliber magazines, meaning they only take sr25 mags.. BUT, I do know that Magpul does make SR25 Airsoft Pmags, just have to find them

I agree, and I'm not saying that it's not accepted, more like the opposite ! Although in Danish MilSim, a DMR rifle is mostly SR25's or M16s that has been modified..

Although I do like your Tanaka, I doubt this is the right place to post it ? AFAIK that's not a DMR rifle ? Anyhoot, it's beautiful !