I probably should've posted this theory a long time ago but whatever!
I finally figured out why Poof and Sparky aren't in Season 10! That's because they both don't rhyme!
Season 10 added a new main character, Chloe who is a new main character who is a girl and a human and not a fairy and a boy like Poof and Sparky.
If you count the 4 main characters we have this season (Timmy, Chloe, Cosmo, Wanda), there are two humans and two fairies and two boys and two girls meaning they rhyme so yeah, if Poof and Sparky are there, they wouldn't rhyme at all so it makes sense for them to remove them because they want the main characters to rhyme.