If you consider the price changes in Bitcoin retrospectively, you would realize that the term 'cheap' doesn't really apply. Every spike in price does not make Bitcoin expensive, it's only higher in value than it was some time ago, but as mk4 said, if you consider it to be lower in value than it could be in a future time range, then it is still a good investment.
Investors may have seen spikes like $300-$1000 and assumed Bitcoin to have become expensive and choose to wait for a correction, meanwhile the market rode that waves to as high as $20,000.
The major reason of investing in Bitcoin is massive buying by institutions that motivates general investors as well . The second reason is Central banks have printed massive amount of fiat currency due to mitigate the impact of Covid-19. The smart Investors believe that BTC is a good hedge again Fiat as it is Deflationary Currency.
I do not know the reason behind the good market o bitcoin
The market price is connected to the fundamentals of Bitcoin. Bitcoin has qualities which an average Joe can think; 'I would want to own one (or a fraction) of that because of <inserts unique quality>', and an investor who may not be interested in those qualities could still want to own Bitcoin cause they feel the demand for it would increase making it a wise investment for profit.
The more demand grows the moe valuable it gets.