Jonathan Harris was not "forced" to play the role as a comedic coward.  That was his idea.  In interviews he often recounted the following.  (Please note I am paraphrasing.)

Harris realized if he continued to play the character as a truly "dangerous"and "cold" adversary, odds are a story would be scripted that would have depicted his demise.  So, he started to ad lib a "wry" line here and there.  One day producer Irwin Allen came onto the set and approached Harris.  Almost glaring at the actor, Allen supposedly said, "I know what you're up to, Harris!  Keep it up!"  And with that, Allen urged the writers to cater to Jonathan's comedic strengths which resulted in the series veering away from the thme of "the Swiss Family Rbinson in space" to the "Smith and Robot" show.

Harris knew he had a cushy gig an he wanted to keep it.

What was there to like?  You mean about Hartnell?  Well, you gotta' understand the the original gol for the series was very different from what it eventually became.  You may have heard or read the term "edu-tainment" to describe a production that's meant to educate as well as entertain.  Though it was decades before that term was coined, it could have been used to describe series creator Sydney Newman's original plan for the series.

Though titled "Doctor Who", the mysterious elderly man was not intended to be the "hero" of the series.  He and his space/time conveyance were to be primarily the catalyst for the adventures.  Barbara Wright the history teacher and Ian Chesterton the science professor were meant to be the real leads.  If the Doctor took them to the past, Barbara's knowledge of history would help resolve the issue, say, by urging Columbus to continue his voyage rather than turning back for Europe.  (Think of a serious version of "Peabody and Sherman" from the old Bullwinkle cartoons.)  If the setting of the adventure were the future or somehow revolved around technological progress, Ian's skills in science would help save the day, like helping a society rediscover the principles of electro-magnetism to generate electricity.  Susan was meant to represent the voice of the audience, asking the questions the viewers would likely ask, like, "What would have happened if Columbus turned back?  Who would have discovered the 'New World' then?"  Or, "How can a magnet and a length of copper wire power s city?"

What Sydney Newman did not want was "cheap, sci-fi thriller" with things like tin plated robots and BEMs (bug eyed monsters).  But due to story development issues, an outline was bumped forward which Newman had hoped to avoid, a story set in a post atomic war wasteland featuring its survivors.  Producer Verity Lambert saw potential in the story despite Newman's initial objections and fought for its creation.  Those "survivors" were the Daleks.  And while they were effectively the tin plated, robotic bug eyed monsters Newman wanted to avoid, he could not deny the ratings success.  For the following six weeks (that serial ran 7 episodes), more families tuned in to see how Barbara, Ian, Susan and the Doctor would escape and hopefully vanquish these "trundling tanks of terror"!  From that point, more adventures involving "flights of fantasy" were scripted, at first alternating with "historical" dramas and eventually replacing them.

It may not have been as obvious when the character Susan left the series, but with the exit of Barbara and Ian, the notion of the series continuing to serve as "edu-tainment" wafted away in the breeze.  The Doctor had become the focus of the show as his was the only character to remain.  And with Hartnell's departure, even he was "reinvented".

Sincerely,

Bill

Tempt the Hand of Fate and it'll give you the "finger"!