Because in this show the character apart from Dixit family ones are ended forever. They don't show them once their track has met its fate & Dev ended that character their only so, considering the history I don't think it will come back.
Remeber Natasha and her dad, Khatri, Ranveer and his family, Ayan and his family. These were the possible villians of the show but they didnot show their villainous side for more than 2-3 episodes at its peak.
Come on who needs a villain from outside when Ishwari and GKB are greatest villains themselves. unlike other makers who add temporary villains but make them permananet with stupid tracks thus ruining the whole storyline for ever. Since show is reality based, So, villains are more likely to reside in our families rather than from outside.
And if CVs introduce Ayan again (highly doubted), then I want Khatri back. atleast he will divert Ishwari's mind to his own.
And as for what was the need to introduce him at the first place then I think that was because they needed to show
1. Sona being an observant for Riya's possible marriage, so that her life isn't ruined.
2. Dev being supportive of her despite his family's adamant behaviour on fixing the alliance.
3. Had Ayan not turn villainous and blackmailed Dev, then how would have Ishwari for the first time had taken Sona's stand.
4. To strengthen Dev's justification for hiding the pregnancy truth from familyas he could see Sona-mother bonding.
5. Ishwari opening up to Sonakshi.(major part)
6. And last my favorite, how could we see fiery Dev who would go to any length to protect his loved ones.
CVs of this show are very unpredictable and smart, they want to believe us one thing but then that turns out to be some other way. and unlike other shows they don't want to ruin the plot with Ayan coming back and then the old ghisa-pita track of Dev's barbadi, Sona helping and then she realizing. Come one she needs to see the truth without ruining Riya's life in that process. Let her ruin everything herself for everyone and then Dev must get back to her for all she did.
Purely my opinion, other views are also appreciated. and no bashing please.