See originally the WWE had one PPV every month with each brand having a PPV every other month. For example, Raw would have Backlash in April but then wouldn't have another PPV until June with Bad Blood or Vengeance. Then Smackdown would have Judgement Day in May and Great American Bash in July this layout allows for feuds to develop longer and have more PPV quality matches on TV. In my book the smaller the PPV schedule the company has will have a trickle down effect on other aspects of the company for example it would give feuds more time develop which allows fans more to get interested in feuds rather than having a feud go for 3 months and then ending it.
The difference between the first time around and the second time around is that the WWE didn't have the WWE Network. I believe that the only reason why WWE upped the PPV count is because they just want as much content as they could which is negatively effecting the product as it overexposes talent and makes new feuds easier to go through faster cause of the extra content. A PPV is supposed to be a special event where we as fans see matches that we don't normally or wouldn't see on regular television. Having a short gap between PPV's makes it very difficult for the company to have enough meaningful matches set up for the PPV, and not having meaningful PPV matches hurts the credibility of a PPV event.