Why can’t use 2nd gen pencil with 8th gen iPad?

The first and second generation Pencil use an entirely different pairing and charging interface. 

The second generation Pencil can only be used with iPad models that have the magnetic/inductive dock that is exclusively designed for the Pencil. Charging is controlled via the iPad magnetic dock.

The first generation Apple Pencil uses the physical Lightning port of the Pencil and host iPad for both pairing and charging the first generation Pencil. The first generation Pencil can also be charged using an inline charging adapter and the iPad’s Power Adapter. This pencil can only be used with compatible iPad models that have a Lightning connector; with the sole exception of the new iPad, that requires use of a special USB-C/Lightning adapter, models of iPad that feature a USB-C port lack any means by which to charge or pair with a first generation Pencil.

In summary, the Apple Pencil models are not interchangeable - and can only be used with the iPad that has appropriate internal hardware.

Here is the definitive compatibility list for Apple Pencil and iPad:

Apple Pencil compatibility - Apple Support

No.

They also pair diffeently too.

1st gen Pencil uses the Lightning connector to Bluetooth pair.

2nd gen Pencil uses the magnetic, inductive connection for Bluettooth pairing.

Also, the 6th, 7th, 8th and 9tn gen base model iPads have NO flat edge to rest a 2nd gen Apple Pencil onto to be able to magnetically, inductively charge, as well, as NO internal charging magnets for charging a 2nd gen Apple Pencil.

All base model 6th gen to the new 10th gen iPad models use the 1st gen Apple Pencil.

This is a fact and technical/hardware spec that will never, ever change.

It is what it is and Apple is never going to go back to older model iPads to “fix” or “correct” this.

Apple made the new 10th gen iPad what it is.

They aren't going to “fix” this model, either.

Sorry & Best of Luck to You!

I’m not enough of an expert to know, but given that there is an adapter for 10th gen ipads anything is possible.

If you don’t get a specific answer here you may with to google non-official sites to see if anyone else has any info or speculation.