为什么我不能用pip安装cudatookkit=10.1，但可以使用conda安装？

Question

$ pip install cudatoolkit==10.1
Defaulting to user installation because normal site-packages is not writeable
ERROR: Could not find a version that satisfies the requirement cudatoolkit==10.1 (from versions: none)
ERROR: No matching distribution found for cudatoolkit==10.1

失败了，但是conda却可以安装：

$ conda install cudatoolkit=10.1
Collecting package metadata (current_repodata.json): done
Solving environment: done

## Package Plan ##

  environment location: /data0/congmin/miniconda

  added / updated specs:
    - cudatoolkit=10.1


The following packages will be downloaded:

    package                    |            build
    ---------------------------|-----------------
    ca-certificates-2021.5.25  |       h06a4308_1         112 KB
    certifi-2021.5.30          |   py38h06a4308_0         138 KB
    conda-4.10.1               |   py38h06a4308_1         2.9 MB
    cudatoolkit-10.1.243       |       h6bb024c_0       347.4 MB
    openssl-1.1.1k             |       h27cfd23_0         2.5 MB
    ------------------------------------------------------------
                                           Total:       353.1 MB

我需要用pip来下载，因为我想把它放在requirements.txt.中