Immediately a loan is been taken, it put the taker under pressure of repayments and when they get into an investment under pressure, most results come out unsuccessful because the pressure of not wanting to lose that money might make the investor exit the investment too quickly for any slight uncomfort he feels which could just be a correction that'll bounce back after.
Bitcoin is a high risk investment and not advisable to take loans to invest in high risk investment. You might do it and become profitable but it shouldn't become a constant habit as at some point the market could dump then you get stuck on repaying a debt that wasn't of use to you which is very difficult to pay.