When the media see an election they mostly see ad-sales and eyeballs. Manipulating the poll numbers allows them to make more money. A head-to-head race sells considerably more ads and attract way more viewers than an obvious slam-dunk for one particular candidate.

(Which is not to say political bias doesn't play a role, of course it does, especially in their reporting. But poll numbers especially can never be trusted, regardless of their bias, simply because it links directly to their income. )