Well the reason in general is lack of Sentai footage.
I guess to look at it from a universe point of view many of these evil encounters are occurring outside of the previous Rangers' hometowns, and said Rangers have moved on to other endeavors in their lives.
Initially the powers given by Zordon were destroyed, yet later restored. Andross and the other Rangers from Earth are still out in space. The Quaser Sabers were placed back after Scorpius' forces were destroyed yet can be obtained at any time. I think the Ninja Storm powers were destroyed. Lightspeed took their morphers back. Wes and Eric are still with the Silver Guardians protecting their city.
The continuity is all over the place with who made these powers and some are technological, some are magic, some are man-made. Others have been around for decades prior to Rita's release and the whole thing began. It gets messy with Zordon and others with what they've been doing for all these years.
I guess that and what previous posters said makes sense. Something like the MCU, explaining why the Avengers aren't taking care of every threat.
With the lack of Sentai footage, it makes even more sense, but I figured there'd be more variety in rangers working together to protect the Earth. The disjointedness feel honestly seems like a copout lol.
Thanks for the good explanation though. ---
It all just disappears, doesn't it? Everything you are, gone in a moment, like breath on a mirror. - The Doctor