It depends on what constitutes "huge" over decades. sure the sum over 20 years looks big but when viewed as an annual spend its not huge by any stretch of the imagination.
Its not clear if you mean a couple of million spent on riders each year is the figure per team or in total but at an an average of around 150k per club per year (low) or 280k per rider (high). If it is the former and being on the low side even a figure double that, is not remotely big.
By and large todays promoters spend what income they generate from gate receipts, sponsorship and media deals and cover small losses (200k is small) as their interpretation an an investment. sure it keeps the sport going which we should be thankful for, but in no way does it create the prospects for growth.
What's lacking is additional working capital (cash) investment by the promoters in significant marketing & PR (so folk do know who the riders are), and the improvement of the meeting experience which (besides a few exceptions) is woeful with sub standard presentation.
The reason its not done is for two linked reasons. 1, The promoters don't know what great promotion (marketing & PR) is and 2, They don't have the money and so rely on keen and willing volunteers to help out rather than being able to invest in engaging full time professional PR, marketing & sponsorship agencies.
Also, worth considering that there is a need to promote at the national level (BSPL) and at the local level (individual promoters) in a coordinated and cohesive manner at a much greater level than is currently undertaken.