I wouldn't say JRPGs are reviewed to harshly. I would say that they have been less prone to the rampant ratings inflation that other genres have suffered.
I think any enjoyable game should get a 6 or 7. Games that are among the best of the year's crop of games should get an 8. 9's should be reserved for games that will be remembered for years to come. 10's should be reserved for games that will never be forgotten.
Anyway, the question is, why don't JRPGs see as much ratings inflation? I suspect it's because having a more niche fanbase results in less pressure on reviewers to produce higher scores, both from the fans themselves and from the publishers themselves.