Well think of it this way Mr.
>>4, compared to OVAs TV anime has a lot more boxes to check before it's green-lit for TV.
Each episode will have to run for approximately 20-30 minutes. Meaning the show will have to work within that time period whether that's too much time (Resulting in unnecessary padding) or not enough time (Resulting in having to be split into episodes). This can ruin the flow of the original work or storyboard.
It will also have to span over a set number of cours. Meaning the number of episodes (Which in reality is how much airtime can be bought) will once again have to either be crammed or stretched to fit this limitation.
As covered, the fee to get a slot on TV will somehow have to be paid for too. Originally, this was not such a problem as late-night TV slots were relatively cheap (Hence why most anime aired late at night), however now that the market is saturated, this is now a problem. The TV channel will have to be convinced that this show is going to get ratings and that the advertisers will be happy with that. Once again this comes down to money; how can you prove to this TV channel that your amazing original series that is both fresh yet enjoyable will be a hit and get them the ratings they need? There's another cute girl/childrens/light novel adaption who can fill that gap, and there is more proof they will sell no matter how good you might think yours is.
It's business sense that they will choose the familiar show, it's their money. They don't really care about anime or the artistic merits of the medium; they just need to make their moneys.
With OVAs, there are none of these concerns - the creators don't have to meet anyone's standards or approval. There are no production companies breathing down the necks of the artists looking for a character they can pitch to the character-goods companies to cover costs. OVAs can be as long or as short as necessary for the story to be told, with any variation of duration.