I think it is mostly because high BTC fees. Honestly, right now rather than buying more bitcoin for 1000 dollars, i rather buy some promising altcoin.
Of course the main reason is also the fact that more and more people are coming into bitcoin, or some BTC holders due to the fear that BTC will fall are buying some promising alts... All of these reasons are responsible for these bullruns we seen last days.
I think the fee's are partly at fault, but bitcoin is reaching such prices that some are diversifying. I also think some are trying to enter crypto with a little less money.
Fees might be seems high for the all online wallet users it is around 20 USD per transaction from the time bitcoin crossed 10k USD. So try to move desktop wallet which can be modified your fees according to amount you are transferring.
If you have cheap value in the wallet your fund will bear for fees alone you will not get the money for that transaction you made with the cheap amount.