I believe that Pelosi & Co are hoping to create confusion by not sending the articles to the Senate, with their end game being a constitutional crisis.

Here is at least one source stating that the Senate can acquit even if they never transmit the articles:

https://www.breitbart.com/politics/2019/12/18/pollak-senate-can-acquit-even-if-house-doesnt-transmit-articles-of-impeachment-constitution/