There can be plenty of reasons for this. It also depends on who the heel is thrown up against, as well as the mic skills and cockiness shown by the new heel. For instance, Matt Morgan, Snitsky, Heidenreich, and other WWE heels have just completely fallen flat.
However, there have been plenty of successful heels who have really skyrocketed recently. The Miz is a great example. After the breaking up of Miz and Morrison, The Miz was promptly thrown into a program with John Cena. After Cena "forfeited" multiple matches against The Miz, Miz would come out and brag and boast about his undefeated record against Cena. Even Cena haters wanted to see this cocky new singles star just get humbled and destroyed. However, although John Cena did beat The Miz, he also helped The Miz look like a fully capable main eventer.
Sheamus is another great example of a huge push early. After coming to the main roster after decimating the ECW roster, he destroyed Jamie Noble. After winning the WWE Championship in what was seen as a "fluke" in a Tables Match against John Cena at TLC 2009 as well as escaping the Royal Rumble 2010 due to a disqualification win, many just wanted to see this cocky newcomer finally beaten. After losing the title, Sheamus went on to a program with Triple H, which ended in Triple H being sent into "retirement". After his 2nd WWE title win/loss, Sheamus snuck into the King of the Ring due to another "sleazy" circumstance (with a 2nd round bye due to the double count-out by Ezekiel Jackson/Drew McIntyre). Fans just want to see the sleazy King get what's coming to him.
However, with the faces, they have to build up slowly. Outside of flashy moves, only one thing will get faces over: win after win after win, convincingly, moving slowly against "jobber" heels, then climbing the ladder to get some convincing wins over mid-carders and upper-mid carders. Example: Jeff Hardy. After returning to WWE, he slowly built up on his own, capturing the Intercontinental Championship multiple times. Steadily working through the mid-card, he challenged Randy Orton for the title at the Royal Rumble thanks to an upset of Triple H (while the IC Champion), and showed a great effort in a losing match for Hardy. Another great example: John Cena. He started strong against Kurt Angle to put himself on the map, then feuded with mid-carders such as Y2J and Big Show, won the US Title, then continued to build up with the occasional main-event feud.
Simply put, it's easier to hate heels than love faces.