Why is it easier to make a great Twitter client for Apple's iPhone than for Google Android phones like the new Verizon DROID? After Robert Scoble wrote a typically impassioned post entitled The Droid fails AS A PRODUCT when compared to Palm Pre and iPhone, and used Twitter clients as an example, Thomas Marban of Android's premiere Twitter client, Twidroid, responded:

one of the main reasons why UIs are unequally inferior are not only the way you build apps (open vs. closed hw/sw system) and the SDK itself but also marginal to non-existing UI standards, no ready-made drag & drop UI items, variations in carrier- & device firmware, hard- & software input, screen sizes, international customizations, modded phones, rooted phones and last but not least completely different expectations among users and the linux'ish target group itself. in a nutshell: beautiful mess. obviously, all these reasons eat up a huge pile of time that one could better spend with improving UX and polishing the interface. those who started early with android development have learned and are still learning it the hard way, just like they did with win 3.1 back in the days.

John Gruber of Daring Fireball, in Lots of Excuses comments:

That doesn’t sound like someone who plans to ever ship something of the caliber of Tweetie, Birdfeed, or Twitterrific. From what I’ve seen of Twidroid, it’s not even as good as Craig Hockenberry’s original version of Twitterrific for iPhone, which was written as a jailbreak app before the iPhone officially supported third-party software. If Android hardware diversity is already a problem for third-party developers, it’s only going to get worse.

This also highlights the advantages Apple has given iPhone developers. Not only is the iPhone based on OS X, but the development tools are based on Xcode and Interface Builder, and while not as many developers are likely already familiar with Cocoa touch as, say, developers might be with Android's language(s) (or web developers may be for the Palm Pre), existing Mac developers can make those tools sing. And, given the SDK Apple provided, even new developers get a huge head start in terms of functions and user interface elements.

Sure, that means there's a lower barrier of entry to creating poor iPhone apps, but it also means great developers aren't wasting their time re-inventing UI wheels, or fighting the OS to do right by their users. They investing that time in their great apps.