Etydhv

In the end of (1980s) Star Trek II: Wrath of Khan, as the Enterprise escapes the nebula, leaving the dying Reliant behind, we see Khan engaging the memory banks of the Genesis device.

Later we see Spock's coffin torpedo ends up on the newly formed Genesis planet and as part of the result of the Genesis effect, clearly gets resurrected.

Wouldn't Khan have been resurrected too? Although, yes, Reliant was destroyed by the Genesis wave explosion, shouldn't he (and his crew??) have been resurrected as a by-product of the Genesis effect as well?

As observed in this dialogue from the movie:

McCoy: Dear Lord. You think we're intelligent enough to... suppose... what if this thing were used where life already exists?

Spock: It would destroy such life in favor of its new matrix.

Khan and his gang were literally at ground zero supplying the detonating Genesis Device with raw material. Spock came in much later, comparatively, to be exposed to a slow "simmer" of the cooling Genesis Effect.

The Genesis Device detonation didn't resurrect anybody.

Spock was very definitely dead when he was put into the torpedo. His death in the engine room was caused by his efforts to make sure the Enterprise escaped the Genesis Wave. His funeral and "burial at sea" was after the sun and planet had formed.

From The Wrath of Kahn (emphasis added):

KIRK: We are assembled here today to pay final respects to our honoured dead. And yet it should be noted that in the midst of our sorrow, this death takes place in the shadow of new life, the sunrise of a new world, a world that our beloved comrade gave his life to protect and nourish. He did not feel that sacrifice a vain or empty one... and we will not debate his profound wisdom at these proceedings. Of my friend, I can only say this. Of all the souls I have encountered in my travels, ...his was the most ...human.

It's uncertain if the Enterprise was was aiming to put it in orbit of the planet, have it incinerate in the atmosphere, or launch it into the star. The survey crew from the Grissom was clearly surprised to find it intact on the surface.

From The Search for Spock:

SAAVIK: Metallic mass.

DAVID: Close-range scan. ...A photon tube! ...Gravitational fields were in flux. ...It must have soft-landed!

ESTEBAN: In code to Starfleet. 'Captain's Spock's tube located on Genesis surface.'

The out-of-universe reason is that Ricardo Montalban was offered the chance to reprise his role (including in a guest appearance in TNG that would have been revealed as a holodeck episode) and turned it down.

Due to worsening injuries from a riding accident in 1951, he could only walk with great difficulty (as you can see at 0:13 of this ad from 1983) and, by the mid-1990s, was confined to a wheelchair.