I'm thinking about why FM-X can not be integrated in programming Drum-key in a Drum Part?

Not sure what you are thinking, but Drum Kits are made up of AWM2 audio samples. FM-X tones do not allow themselves to be easily mapped to a single Key like an audio sample (there is a huge difference in how each technology generates its sounds).

When initializing a Performance, you can choice between four Types:
Init Normal AWM2 Part
Init Normal FM-X Part
Init Drum AWM2 Part
Multi/GM 16-Parts

A basic understanding of the AWM2 sample based engine and the FM-X engine should say to you why this is not possible in any of these types. Each Part can be either AWM2 or it can be FM-X, never both simultaneously. Normal instrument plays scales across the keys — all keys are typically apart of the same instrument; Drum Keys each have an autonomous instrument with its own set of parameters.

The Normal AWM2 (samples) Part is made from 8 Elements (8 Waveforms)
The Drum AWM2 (samples) Part is made from 73 Elements (73 Waveforms)

And although you can build Drum sets using the Normal instrument layout, as in the “8Z” (eight zone) Drum Kits found in the Factory Presets, conversely creating a Normal tuned instrument using the init Drum Kit is not practical.

The FM-X, as a tone generator system, does not lend itself to be mapped to the Keys of a synth in a similar way as to how Drum Samples are mapped to a Kit. It is an entirely different approach and system. Entirely.

FM-X is highly capable of generating Drum and Percussion instrument sounds, however — in fact, the tone generators for early Yamaha Electronic Drum Kits circa 1980s, were exclusively FM engines. Back then each set of Operators were used to generate one drum/percussion sound.

If you are doing sequencing with multiple Parts, creating an FM-X Kick drum, for example, would mean dedicating the Part to FM-X.
It is not possible to combine AWM2 and FM-X in a single Part.

So we can use sample based drum sounds as well and FM-X sounds parallel.

You can do that now. In fact, that is exactly what you MUST do... in parallel Parts.

Current system allows for a great deal of interaction between the AWM2 and FM-X engines via the Side Chaining functions and the Envelope Follower. This allows a traditional drum/percussion instrument to be used to modify or modulate an FM-X tone... they can be combined to sound together, or the drum sample can be disconnect from the audio output - use just its audio energy to modify an FM-X creation.

When compared to previous pairing of AWM2 and FM (where samples were used within the FM algorithm), this new method of interaction opens many more musically useful doors. We invite you to explore the currently offered functions

Extra Credit:
There are several awesome FM-X Drum sounds available... check the EasySounds (FM-X) stuff. If you are looking for punchy, in-your-face Kick drums, nothing, is better than FM-X, trust me.

Idea was submitted to ideascale some time ago (2018): https://yamahasynth.ideascale.com/a/dtd/Add-FM-Drums-to-FM-X-or-Successor/217280-45978

In 2020 - someone else duplicated the same idea. The idea is "on the board", so to speak.

I'm sure a company that can design and manufacture leading-edge musical technology can "make it happen" despite the obvious and known differences between the technologies. It's not an easy no-brainer kind of change. It's a good challenge for the powerful engineering minds at Yamaha should they take up the effort (how to maximize the resources to retro-fit FM-X technology into a per-key system. Having one algorithm configuration at different pitches is "easy" - having other options requires some creativity). Despite having submitted the idea - I have no expectation of its priority vs. other competing ideas.