It's pretty simple really. The shows you named as being released as complete series sets are very well known and  a large enough of a fanbase who are willing/can afford to purchase complete series sets.

Universal released the first two seasons of Leave it to Beaver and then due to poor sales performance they didn't release additional seasons. It's only because they sub-licensed the series to Shout that seasons 3-6 are getting released. Shout is putting out the complete series set before the remaining seasons because there are fans who felt they have been burned by Universal and will only purchase the series if it's released as a series set. The same thing was done with G.I. Joe & Transformers. Complete series sets first, then season sets.

I believe Mr. Belvedere & Simon & Simon are currently being released via Shout Direct due to a lack of sales at retail stores. Fans may have been purchasing the sets, but the typical TV on DVD consumer was not. Be thankful that Shout is even releasing additional seasons via their webstore because there are many other Shout properities that are still incomplete.

Shout follows the same business model as other labels, they release the first season and if it sells well enough they release the next season. The cycle continues until either the entire show is released or the label stops due to lack of sales.