Basically what I've seen is that conservatives turn out in droves to vote and liberals are more cynical and apathetic about the system and don't bother to, hence why they have control more often than not.
That's over simplifying things here. It's not that we have a more conservative electorate. Honestly, even though the electorate was whiter and older, politically they're still about the same as 2012: rational, centrist people.
1. A lot of states that had Senate elections were in states that are trending more and more Republican like Arkansas, Louisiana, and West Virginia. Even if this were a good Democratic year, those states would have fallen Republican anyways due to the changing demographics of those states. If there were more Democratic states on the map, the outcome wouldn't have been as harsh.
2. People don't care about social issues anymore. The social wars are over and the left has won. The only way social issues are going to influence an election is if a candidate makes a bad mistake like Todd Akin talking about what counts as legitimate rape or Mark Udall trying to make it look like he will make birth control pills rain down from the sky like they're Skittles. A candidate who focuses far too much on the social issues in today's age, left or right, is going to lose.
3. The Democrats were the ones with bad candidates this time around. The GOP learned their lessons from 2010 and 2012 on how bad candidates will sink a ship. The Democrats on the other hand decided to run candidates like Charlie Crist, a person who changes political affiliations constantly for his own purposes; Martha Coakley, the lady who lost Massachusetts' Senate seat and thought that Kurt Shilling was a Yankees fan; Bruce Braley, a guy who looks down upon farmers while running in a state dominated by farmers; and Mark Udall, who took a guaranteed win and squandered it by trying to make his opponent look like a pro-life extremist and focusing almost exclusively on that.
4. The Democrats got cocky and complacent this time around. They put far too much faith on their GOTV efforts while thinking that the GOP did not learn their lessons from 2012. When the Democrats looked at the North Carolina early vote, they assumed their lead was very good while ignoring the demographics of the early vote and forgetting that many North Carolina Democrats are legacy Democrats (or DINOs). They even took Virginia for granted, a state that is solidly purple, not a state that is trending blue. It also didn't help that the Democrats ran on some VERY BAD data in their calculations.
5. Because of their focus on building a coalition of minorities, youths, and single women, the Democrats now have some major demographic issues of their own. Just like how the GOP has issues with women and Latinos, the Democrats have issues with whites, men, and married women could be a problem for Democrats in the near future. By focusing on the Obama Coalition (a coalition that has been left behind in the Obama Recovery), Democrats have alienated important demographics. And unlike Republicans who can survive without the black vote, Democrats cannot survive double digit losses with men without double digit wins with women. They cannot survive massive losses with whites when minorities historically have poor turnout.
6. This comes down to the issue of governance. While the economy has improved a lot since Obama has taken office, the Democrats really did not do a good job in messaging that. Not only that but there are a lot of people who feel left behind in the Obama Recovery which hurts the Democrats. When the government bungled the initial responses to ebola, ISIS, Russia, the IRS scandal, the Obamacare website, the VA scandal, etc. it makes the government look incompetent. And when the government looks incompetent, voters are going to take it out on the party in power, in this case, the Democrats. This isn't about Obama's policies or ideology, but more along the lines that the Democrats did not look like they were good at governing and making things run smoothly. This transcends things like a more "conservative" electorate.
7. The GOP learned their lessons from 2010 and 2012. The GOP and their allies such as the Chamber of Commerce worked to ensure that no stupid Tea Party candidate was going to sink them this time around, even in extremely conservative states like Mississippi and Alabama. They ensured that their candidates would run tight and disciplined campaigns. They vastly improved their data management, GOTV efforts, outreach towards women and Millennials, etc. The fact that the Democrats acted as if the GOP didn't learn their lessons from the past two election cycles is absolutely mind-boggling.
8. Midterms are just naturally difficult for the party in power. Simple as that.