SNCF should be able to book both these trains. You should be able to book by phone:

https://www.sncf.com/en/customer-service/contact-us/telephone

Hey,  

I can make reservations for both trains via interrail on the date you want. So be sure to check again.

For the night train, which leaves Paris at 10.22 PM, I can only book seats via Interrail. So you will only be able to book this train via SNCF. 

@Nanja Why can you only book seats for the night train on this connection with your reservation service?

Hi Sven, unfortunately there is not a simple answer to your question. This is often something decided by the carrier and can have various reasons. One of them could be the prevention against fraud as the carrier is not able to check certain details which would be needed to have the seat reservation option available on their website. Cheers, 

The question was: how come for the SNCF night trains only seat reservations are possible via Interrail.

Apart from that, not being able to check certain details required for seat reservations sounds very much like an excuse for "we can't be bothered” :/

Thanks for your feedback rvdborgt.  I am sorry to hear it comes across like that as it is not meant to be an excuse. Just to clarify, we are always making sure that if another seat reservation is or can become available, we will inform our customers on our website. Regarding details on why seat reservations can only be booked in a certain way cannot be answered in a simple way.
​​​​​​​In total there are quite some seat reservations from various carriers involved in the Pass. The most updated information on which ways are available to book seat reservation is most relevant. That said, we do very much appreciate you sharing your perspective with us, so I will take along that there is a need for such details to be shared and will investigate further internally. Cheers, 

I just realised I may have been unclear. I meant to say that it sounds like an excuse from the operator's side, who, let's be honest, are not always very interested in Interrail passengers...