That is not a trend, the number has actually decreased. State wide elections are trending more republican than before.
Maybe Wendy Davis was a fluke. Her support was largely leftists outside Texas, and I honestly don't know if a better candidate would have reduced Abbott's margin.
But, we can look at the outcome of past races. Each set of percentages is R, followed by D.
2014: 59.3% - 38.9%
2010: 55.0% - 42.3%
2008: 39.0% - 29.8% (two other candidates took 30.5%)
2002: 57.8% - 40.0%
1998: 68.2% - 31.2% (GWB's re-election)
1994: 53.5% - 45.9%
1992: 46.9% - 49.5% (Clayton Williams' gaffe)
1986: 52.7% - 46.0%
1982: 45.9% - 53.2%
I don't see a discernible trend, but there aren't enough data points.
Texas politics have changed drastically in the past three decades: Conservative Democrats have either switched parties or been replaced by Republicans, and the remaining Democrats have been moving farther left.
It's also worth noting that a Hispanic woman was the Democrat candidate for Lt. Governor in 2014. If the Hispanic vote was truly a factor, then she should have benefited. But, that race was 58.1% - 38.7%, almost the same as Governor.