For some Power Rangers fans, they do also like Dekaranger (I think most of them saw SPD first). I think are are some of the reasons:
I think it has to do with the cast of characters. IMO the Dekaranger cast is more well-balanced than the SPD team with how each one's strengths and weaknesses are shown and balanced throughout the whole show. Each one of them had funny moments, even the serious guy like Hoji Tomasu who I felt triggered every positive and negative emotion of the viewer more than the others. Some of them I felt were so MMPR in personality like Hoji Tomasu, Jasmine Reimon and Tetsu Aira. Also I think the hot babe Ayumi Kinoshita may have played a role in it too.
I think it has to do also with how the script was written and its one-of-a-kind storytelling which is kind of different than the typical PR/Sentai expectation. There was the right blend of seriousness with humor. Timeranger and Time Force anyone?
Some fans didn't like the costume of the American counterpart but that's just plain shallow. Some liked the American counterpart costume better. Go figure.
The show itself had more bad-ass police action with less CGI effects. That was kind of present in Saban's era of Power Rangers. Hoji Tomasu's suitless fight scenes rock. Some friend of mine commented that he sees Hoji Tomasu like Jason Lee Scott of MMPR. I agree.
I wonder if the unique villain Aburera has something to do with it. He operates ALONE and goes from one random client to another, which he indirectly controls them. On the other hand, Power Rangers SPD seems to stay on the stereotypical side. He didn't appear so often yet he was so feared by the group because of his influence.