Despite the having exciting and intriguing storyline, many of the Doraemons Stories were not adapted for animation unlike the original manga or the movies. Here are several possible reasons.
1. Voice Actor's Schedule
As the main series was produced weekly, approximately 52-60 episodes per years including special episodes, the voice actors are required to do the recording every week. In addition, there is also a two-hours long film produced every year (which took almost a year long to produce) which equals more recording time. Added up to the fact that most voice actors also works with more than one animes (especially the more popular ones) and their schedule would be very busy. Well, here goes the first reason!
2. Staff and Production Budget
Unlike other anime based on the manga which has 13, 26, or 50 episodes, Doraemon is a series that will keep continuing as long as there are audiences, good rating, and have high box office (for the movie). Staff must be working the entire year to keep episode and movie quality up to standard as well as minimizing the budget's usage as low as possible. Assume that some staff that work on the series also work on the film, then in order to produce The Doraemons series they would have to either create another division (which require hiring more staff) or new studio. Not something practical and can be done in the near future especially when the main series has undergone great animation update (and extra expense), obviously.
Majority of the episodes in the series take place in Nobita's neighborhood such as the Open Lot, school, and the mountain, allowing the reuse of background images for the production for both 1979 and 2005 anime and another method of reducing the budget. The Doraemons, like many of the movies, take place in multiple locations which mean more background must be drawn, more so since the Doraemons' genre is more focused adventure than slice of life.
- For comparison, while some of The Doraemons:Robot School's chapters do take place in the Robot Academy, more than half of the three volume take place outside the academy in various time period. This doesn't include the locations in The Doraemons and The Doraemons Special.
This is more or less comparable to amount of background images used in the movie animation. Definitely not cheap!
Various The Doraemons mangas were published during the time of 1979 anime series and are mainly considered canon to the franchise at time since Principal Terao Dai and the Robot Academy do appear in one of the episodes. However, the 2005 anime series changed some details of Doraemon's birth by removing the scene that Doraemon lost one of his screw during the production due to the Dinosaur Hunters, and that Doraemon apparently never went to the Robot Academy. Bringing the Doraemons back would require retconning these details. Still, that's a little problem compared to the previous three.
The length of the Doraemons' Chapter varies from normal episode length (aka 15 minutes), to one hours special to movie's length. Let's take the Nostradamus Arc for an example.
- Prologue (Nobita and Doraemon in the time Tunnel, change in Present Day Japan): 5-10 minutes
- Christopher Columbus Part: 15 minutes
- Ancient Inca Part: 15 minutes
- Stop Nostradamus and save the world: 10-15 minutes
That's one hour special episode for you. Still, there might still be a way.