There are various different sectors within the share market - they include mining companies, banks, retailers, technology companies... Each sector tends to have a different dividend ratio. For instance, mining companies are spending a lot of money on equipment to enhance their mining operations, so they tend to have low dividends. Banks, on the other hand, don't have much equipment, and tend to have high dividends.
You CAN get indexes that concentrate on different sectors, and there are some indexes that are "high dividend" indexes. If you were to only go after high dividends, you would be concentrating on one portion of the market, so you would tend, overall to get less return.
Also, high dividends can be a problem. Mining companies start out needing a lot of money to set up, so they produce low dividends. When the mine is fully set up, and they can't expand it because they have covered the entire area with their equipment, the mining company may offer high dividends, because they don't need that much equipment. BUT, they are possibly about to stop producing their mineral, so this may be a terrible time to buy the stock, even though it's giving good dividends.