Is there a reason Clapton played lead? Could Harrison not do it?
According to the wiki article, they had made several recordings of the song but Harrison was not satisfied with the guitar. Harrison was a workmanlike lead guitarist, IMO, so I can see that. He asked Clapton to play lead, and the rest is history. It’s an OK song, not one of the cream of the ouvre, I think.
According to Wikipedia, Harrison didn’t like his initial tries at the solo and asked Clapton to try it. I think he also wanted to focus on his singing and rhythm playing. Harrison and Clapton were good friends. Harrison wrote Here Comes the Sun in Clapton’s garden, and he also didn’t seem to care when his wife Patti left him for Clapton. But that’s a different album.
Thanks, never crossed my mind to wiki.
According to the Beatles Anthology, George invited guests such as Clapton and, later, Billy Preston, to force the group to behave a little better and to force John and Paul to take George’s music more seriously. John and Paul tended to be a little dismissive of George’s work.
Blasphemy! It is nothing less than the single greatest Beatles song ever! IMO anyway… 
That song required a dominant lead - one that really stood out. That was never Harrison’s forte at all; the lead in Something is about the most up-front he got with his leads.
Here’s a snippet from Guitar World with Harrison telling the story.
Nothing to add other than this nice story.
On a radio show over here George was interviewed once and was talking about going on tour and Eric going out with him and playing the solo on stage.
Harrison said that since he knew Clapton as a friend etc. he never really thought of him as a rock god or anything like that until the first show live. He said he was playing the song and at the beginning of the solo Clapton walked forward into a spotlight and started playing. George said the hairs stood up on the back of his neck and it was only then he realised that this was the CLAPTON the everyone worshipped and he was only noticing it for the first time. He said it was one of the strongest feelings he had on stage in his carreer.