Maybe people can't understand what you're saying because you're not clear, or you misspeak. You said:
If it's better then RG8 then how's come there isn't a 50ohm version that's the same size?
No one said RG6 was better than
RG8. They said it was better than
RG8X. Which isn't entirely accurate since RG8X is a mishmosh of different cable types/sizes that don't fit into the more standardized RG58 or RG8 categories, as zz0468 points out.
BTW, there is a 50 ohm cable close to the size of RG6...it's called TM LMR-300 or Belden 7809A
TM LMR-300: 50 ohm, 0.300 diameter, -2.4 dB/100' @ 150 MHz, -4.2 dB/100' @ 450 MHz, -6.1 dB/100' @ 900 MHz
Belden 7809A: 50 ohm, 0.300 diameter, -2.2 dB/100' @ 150 MHz, -3.9 dB/100' @ 450 MHz, -5.6 dB/100' @ 900 MHz
Incidentally, the 75 ohm version of LMR-300 has lower (better) attenuation than the 50 ohm version (although they are the same numbers as the Belden 50 ohm version). This is also true of the 75 ohm version of LMR-400, it has slightly lower attenuation than its 50 ohm counterpart.
TM LMR-300-75: 75 ohm, 0.300 diameter, -2.2 dB/100' @ 150 MHz, -3.9 dB/100' @ 450 MHz, -5.6 dB/100' @ 900 MHz
For the record, the losses shown for RG6 in that linked chart are inaccurate.
Belden 9116 RG6: 75 ohm, 0.270 diameter, -2.2 dB/100' @ 150 MHz, -4.3 dB/100' @ 450 MHz, -6.8 dB/100' @ 900 MHz
For comparison purposes...
TM LMR-400: 50 ohm, 0.405 diameter, -1.5 dB/100' @ 150 MHz, -2.7 dB/100' @ 450 MHz, -3.9 dB/100' @ 900 MHz
Belden 7809A: 50 ohm, 0.403 diameter, -1.5 dB/100' @ 150 MHz, -2.7 dB/100' @ 450 MHz, -3.8 dB/100' @ 900 MHz