There was some concern that it would very quickly get included in relevant indices due to its size. That would trigger a bunch of funds that are based on these indices to have to automatically buy their stock to keep matching the index. Those specific funds are often included in 401K plan.
Last I heard that got blocked by the relevant authorities requiring a company to actually be profitable as a prerequisite to being included in these indices.
Edit/PS:
S&P blocked them. Nasdaq and FTSE did not.