I had assumed this to be because their earnings were so small. For example Tesla has an undefined P/E ratio since it's a divide by zero. I had also assumed this to occur when a company sacrificed short term profitability for higher adoption.
EDIT: not sure why the downvotes, but maybe because I'd incorrectly assumed earnings to a non-negative number. Still TSLA's P/E is N/A according to the few sites I've checked:
https://finance.yahoo.com/quote/TSLA/
https://www.nasdaq.com/symbol/tsla