Valve said themselves that these episodes are Half-life 3 sequel in the form of episode expansions. Instead of taking another 4 years to make a sequel they decided to make these episodes instead, these are basically experimental and if sucessful all future content could be like this with no true sequel, also it gives them a chance to experiment with different features. An example would be the new portal game that will ship with Episode 2.
If they worked say like 4 years on a sequel and included a new feature an example of a feature would be the portal gun and if it played a major part in the game and if people didn't like it, it would be a major blow to them where as if they release it with episodes they can say "Ok that didn't work so good we'll try something different/ or improve on it". If the feature didn't work so well would you rather have 10 months work down the toilet or 4 years worth?
Was explained better with an interview with Gabe Newell but can't be bothered to dig it up. So basically he said dont expect a Half-Life sequel anytime soon. It took 6 years for the sequel to the original half-life and Gabe and many others in the company didn't want people to wait that long again.