Caveat: 'Why Not' questions are like 'What If' and rarely have a definite answer. It's an (educated) speculation at best.
In case of interface and connectors, NIH (Not Invented Here) is a big issue. Beside the fact that adding more interfaces costs money, manufacturers usually love to have closed systems where they control - and most important sell all parts. Just think why there is no common remote even though a majority uses the same technology and protocol, they assign the codes in different schemes - often even within the same brand.
Or think the many attempts of computer manufacturers to come up with proprietary connectors for standard interfaces. The most prominent example might be Apple and the switch for RS422 and mini DIN connectors for the Mac's serial interfaces. Sure, there can be many arguments made as being more reliable and so on, but all of them don't really matter in a desktop environment. What counts is that customers will have an easy way of using prepared Apple peripherals or will have to spend more (converter/cable) to connect standard devices.
The SCART story is even proving this as being a reverse NIH that backfired. Developed by the French TV makers' association in the 1970s (yes, Europe had still many TV manufacturers back then :)) as a common standard for interchange, they saw a chance to use it as a protectionist measure and 'supported' the government to adopt it as requirement for TVs sold in France. The 'rationale' was that foreign (non-French that is) manufacturers would avoid the additional cost of making special French-only TV-sets and thus leave them their market - while they could even save money by leaving the connector unpopulated on export units. Weird logic, isn't it?
Except that idea already hadn't worked some years before, when France selected SECAM as the colour standard to be. European manufacturers just modularized their TVs, eliminating next to all additional cost - while gaining more flexibility at the same time - leading to ultimately larger market shares overall and in France.
So with SCART, TV manufacturers started to not only fit SCART connectors to sets designated for France, but to all of them - starting with their top line models. By the beginning of the 1980s, most new TV sets in (Western) Europe featured a SCART connector accepting always composite video but also more often than not RGB.
While many of the upcoming home computers where designed primarily for the US market with a composite output meant to be used with a modulator, a SCART cable became a common add-on in Europe, eliminating the noise due to modulation and demodulation. Even more so for machines offering Y/C (like the C64) or full RGB. And French (Thomson et.al) home computers of course featured SCART out of the box :)) In the mid 1980s, SCART was the de facto standard for a colour monitor. Atari sold their STs in many countries with a ST to SCART cable.
If it's that great, why not in the US? Well, US and European markets were quite separated back then. Beside different technical norms (60 vs. 50 Hz and NTSC vs. PAL vs. SECAM) US customers where not only used to different designs, but also quite different user interfaces (*1). So except for few upper end models there was no common design. Even European manufacturers exporting to the US (quite a few) did set up separate production lines for special US designed models. As a result, SCART wasn't the ubiquitous interface it was in Europe, and thus not really interesting to US computer manufacturers.
Again, if SCART is that good, why did it (mostly) vanish for computer use even in Europe? SCART is tied to TV signals, and while this is good to somewhere up to VGA/SuperVGA, by now it would have been replaced anyway. But the true story is again about closed systems. Professional computers where usually sold as a system. Locked in status was not really an issue, and connecting to a TV rarely a goal. So using a generic interface did not bring any advantage (and was not forced by law anyway) - in contrast overseas manufacturers (at that time mainly Taiwan) could offer lower cost by huge volumes when just offering the most common solution. In this case the VGA connector.
Long story short, an inverted NIH being superseded by huge volume 'it works; who cares'.
Speaking of benefits of legal standards - USB for phone charging tells exactly the same story. People may have already forgotten, but less than 10 years ago next to every phone, PDA and alike had their own proprietary connector for charging, their own wall wart and often another proprietary cable for data exchange. It was due to force of the European Commission that the micro-USB connector became standard for charging of phones. Even Apple has to join by offering an adaptor. Beside reducing the need to carry several chargers, they also became dirt cheap (*2) not to mention that next to any computer can be used for charging. As a side effect the whole mobile business, from phones to tablets to cameras uses micro-USB. what a relief.
By now the Commission's force has run out (such measures are usually timed) - it's time for manufacturers to make the world incompatible again :(
*1 - For example it was even in the '80s still common for a TV to have a dial for selecting channels. Something that already went out of style in Germany somewhere in the early 1960s. We wanted to have function keys to sort the channels the way we liked them, not how they are aired.
*2 - I still remember paying 30 Euro for a replacement charger in the early 2000s - now they are 5 Euro at most and can be bought everywhere.