It's only recently (since early 2006) that Etihad started to introduce their new aircraft. Until then all routes were operated by older leased aircraft, different configurations, etc.
Regardless of any service or consistency issues, lack of FFP (until recently) etc. there's no way they were in a position to compete with other 4-5* carriers in the past.
Now that they've taken delivery of multiple aircraft, phased out many of the leased ones, etc. there's no doubt that rating will rise.