The trouble is when those "implementation costs" are $20k for the base platform, another $10-15k in addons/extensions/plugins/whatever and then $100-300/hr for consultants to set it all up and suddenly you're looking at 6-12 months and $100k in cost to get it going.
That's out of the price range of many small to mid sized businesses. You have to be pushing the higher end of "mid sized" before that becomes a plausible cost.
To compare with Postgres: Yes, if you need a massive database to handle multiple terabytes of data and thousands of concurrent connections, you're probably going to want someone who knows what the hell they're doing to set it up, and it will cost you.
On the other hand, if you just want a couple dozen small tables to be used internally by your 5-person staff, pretty much anyone with even minimal DB experience can get that up and running.
That's the difference -- PG handles basically any size of operation. Yes bigger is costlier of course, but smaller is possible which is not really something you get with SF. If you can't afford $50-100k up front costs plus who knows how much over the long term in maintenance and support, you're left with good old Excel spreadsheets and other hackjob "systems," which is what many many companies still use.
Like I said, I'm sure there are CRMs (including, though not specifically, open source ones) that are more reasonable but none of them have taken enough market share to be really well-known and common in the same way that Postgres and MySQL have done in the database market.