Live Season 2
Because that is how the Devs implemented that feature.
You can always exchange Gold for Gems and get it at no monetary cost. You can also join (in a party) anyone that has LS Season Two, to experience the content, but you will forego any Achievements or rewards.
Good luck.
As each LS chapter is released, if you log in during the time that chapter is active it can be added to your “Story Journal” or whatever, and you can revisit that content later. This is the free part.
After that chapter ends its active period, you can buy it from the Gem Store if you didn’t log in and save it.
Season 2 is long gone and if you didn’t log in and save the chapters when they were available, you have to buy them. HoT does not include them.
Word to the wise: log in and get season 3, while it is active