Tidal - why only remastered versions?

Tidal takes a feed from the music companies.   This feed will contain the entire live (ie not deleted) digital catalogue.   When a remastered album is released the record label generally deletes the pre-existing version of the album.   So at the point at which a streaming service sets up and takes its initial feed from the music companies they will be supplied the latest master.  If a streaming service had already been delivered the older master they will continue to have it on their service if a new remaster is delivered.   It is then up to the streaming service whether they continue to hold this on their service. 
It is generally not possible for a streaming service to access a digital master that existed before they existed but was superseded prior to the streaming service being created.  That is because, as I said at the top, a record label will not deliver deleted products as they are not part of the live catalogue. 

With regard to @mahler123's point re classical.  Remastered classical music sounds better as the music that is remastered in the classical domain is generally older pre 1970s repertoire which can be cleaned up and improved whilst not changing the compression or dynamic range.   @mahler123 also enquirer why it could be a bad thing to be remastered: unlike classical, pop/rock music is remastered with more 'loudness', more compression and a reduced dynamic range. This is to help the music sound comparable with modern production techniques and to sound better in the noisy environment of earbuds and car stereos. Many audiophiles consider this a backward step in sound quality, hence they seek out earlier editions of the master recording.  For CDs this is often the original master tape transfer from the 1980s. 

The take home message is - don’t rely on tidal as an archival source. The same with the record labels. Maybe you guys should hold on to those old cds and records.

@marklindemann
Exactly right. It also explains why the early CD editions of albums command a premium price on discogs.com.

Worth noting also are the digital audiophile remasters on labels such as MOFI (MFSL) and Audio Fidelity: the record labels rarely (if ever) grant streaming rights to the licensing agreement.  Hence they will not appear on Tidal.

@junzhang10 

Isn’t remastered sound better? MQA is also a remastered format, sound much better than original.

Have a look at my post 12 posts up from here.  I explain in detail why remastered is not generally better for audiophile listening, apart from with older classical recordings. There are exceptions of course, but as a general rule a non-remastered album will sound better on a high-end system. 

MQA is not remastering.  MQA is a type of file compression (And confusingly this is completely different from "audio compression" which is the compression that causes remastering to often sound poorer on good hifi).   MQA file compression is similar in principle to FLAC or MP3 file compression, but just a more complex form that a group of people, but by no means all people, think helps improve sound quality.