Fixing or limiting supply of a cryptocurrency isn't enough for its continuous price increase. It's about the good and strong principles governing the cryptocurrency that won't be easily violated. Plus the awareness of what you can create or do with the cryptocurrency under the protection of the good principles makes it even more valuable.

It's like valuing a country so much because of the good law governing it that can't be easily broken.  You are willing to pay high price to invest in the country no matter how small the investment is as it's almost certain that there won't be bad policy changes that negatively affects the investment, and you can rest your mind and focus on the development of the country, if it's not very developed. If something changes against you, you are almost guaranteed to get justice.  
Compare that to a country with unfavorable law that the administrators can change or use against you whenever they want. No matter how attractive the country is made to look, you won't be convinced. You may only be willing to pay less even if so much can be done on the country.

Bitcoin is more attractive due to its strong principles that are hardcoded into its system and very difficult to change. The principles combined are more important in the valuation than just the supply limit. If one or more are broken it won't be valued as much. By the way, the more its valued, the higher the price