Why does it take a long time for heroes?

The reason why it's a lot more frequent is heavily influenced by Riot's business model, it's not truly "free to play" but more "free to play but you have grind for stuff or pay money for it". It's very apparent when you compare the free2play models between Dota 2 and LoL. There's profit to be made in LoL when buying the champions and it's already formulaic with how Riot design champions. Since champions have been forced into set roles like Bruiser/Assassin/Marksman/etc, it makes it easier to design champions to fit these roles because there's a formula of design for each role that has been set since the release of Xin Zhao (maybe even Irelia).

Take a look at Xin Zhao and Wukong, although their skillsets are somewhat different in mechanics they're still incredibly similar in design. Both has a gap-closer, both have a buff or some sort of way to survive, an attack buff then their ultimates who knockup or back. See how similar they are? Both these champions also play almost exactly the same where Xin seems to have more damage and Wukong has a little more maneuverability.

Take a look at the unique champions like Lee Sin and Nidalee, they're awesome to play and felt original while having a relatively high skill-cap. However they're few and far between compared to the churned out champions like Diana who is a slightly different Akali or Talon and viceversa. There's a formula on what to churn out.

Since Dota's hero designs are not tied to a business model of selling each hero, Icefrog can take as long as he wants to design a unique hero. Look at all the heroes we have so far, all of the heroes play incredibly different even when there are some similarities between some spells. Like Sven, Vengeful Spirit and Wraith King, originally back in DotA 1, their stuns were exactly the same while minor value differences. Now they're different and it was one of the problems Icefrog had to tackle to make them different. Another good comparision is Lina and Lion, their ultimates were exactly the same until only recently (like a year ago). However they play entirely different even before the changes, Lina has a lot more damage and a lot less CC while Lion doesn't have as much damage but a lot more CC.

You can just take a look at the whole of Leagues champion pool and compare them with Dotas. There's a lot less variety ESPECIALLY when you consider how League is balanced.