It is a combination of two things:
--The "great deals" thing are to drive human psychology. Software companies have found out that a way you can increase revenues is to sell things at a high price normally but have sales off and on. People then get the "I have to buy this now, it is such a good deal!" mentality and they get something they might not otherwise. Heck, I own Wave Diamond for that very reason. No need for 90% of it but I got sucked in by the deal. Also then this allows them to get more money from people who want something RIGHT NAO and will pay an inflated price. Basically they get more sales and more money than if they just had it at a lower price all the time.
--The overall lowering of price is to keep competitive and to continue to monetize their old plugins. Most of what they sell in their staple "material" bundles are old plugins. Some of them are over a decade old. Even a lot of the newer stuff is still old. A lot of their lauded plugins, Liner Phase EQ, IR-1, etc are over a decade old. I mean ya they've updated the UI a bit in some cases and ported them to 64-bit, but they are old plugins. So how do you keep people interested? Make the bundles more appealing. Drop the price so people find them to be a good deal, buy in, like your stuff, and then want your newer stuff which is something they have to buy separate (or get a more expensive pack).