Because 1 and 2 are ports of the PSP versions, which were updates actually done with care, and 3 and 4 are the DS versions with better textures and a smoother framerate (although I think the UI is ugly as sin) so overall an improvement.
Basically, they spent the time, effort and money earlier for actual gaming systems and just did the basic work to port them. With V and VI since they didn't have something better to throw up there and they knew the majority of people playing games on phones don't give a crap they just slapped a shitty filter on the sprites, threw them on a 3D background and called it a day.
Oh, why must I tell 'em again?
What must I tell 'em, my friend?
That we are born into sin
And we are worshiping murderers
Nothing but evil duplicitous men
I know you heard of 'em all
We should just murder 'em all
We should just murder and ball
We dance on the ruins we looting from all of they palace's walls