Two things: (GPU) cryptomining is extremely unpopular with PC gamers, especially those who DIY build PCs, because the demand for GPUs for mining is substantially worsening the general GPU shortage, pushing prices even higher. NVIDIA has made a lot of noise about how they are on the 'side of gamers' (such as with their Low Hash Rate cards which have a software lock designed to reduce mining efficiency which held for about a year or so), but in general seems to have been happy in practice to make as much money as they can off of it. So NVIDIA has a marketing interest in downplaying the impact of cryptomining on the shortage while selling as much as they can into the high demand and high prices at the moment. (And tech journalists communicating to these gamers do read these reports and tell the consumers about this stuff)

Secondly, from an investors point of view, GPU cryptomining demand is seen as a lot more volatile than PC gaming demand. A crypto crash or the planned ethereum switch to proof of stake could reduce this demand substantially. If a larger fraction of NVIDIA's sales (and thus revenue) is due to cryptomining, it's less likely to be similar next year (not only because cryptomining might buy less new cards, but also because they might flood the market with used cards, reducing demand for new cards from PC gamers, which happened during the last big crypto crash). This is the main reason the SEC is not happy about it. When reporting to investors NVIDIA also has an incentive to downplay crypto-related demand.

Minimizing the perception of downside risk in sales when it dries up to keep share prices higher than they otherwise would be. Had they come out and said approximately X% of a given quarter's sales were mining related, it would have most likely been priced into the stock as cyclical sales with a lower resulting multiple.

edit: this is an old story see previous discussion https://news.ycombinator.com/item?id=31284952

Presumably that the fine will be much, much less than whatever the value of the benefit is that they earned from the omission.