Spo - Rioja is far richer and has bodegas that are quite large, so they're able to do that kind of thing.
They've also been pouring money into marketing and PR, with more misfires than successes IMO.
But all that aside, I don't see the comparison to Jumilla in quality or quantity, other than the fact that they're both in Spain.
The entire region of Murcia, including Jumilla, Bullas and Yecla, is about 40,500 hectares and of that Jumilla is largest with somewhere around 30,000+ or so. OTOH, Rioja is about 64,000. The production figures don't correlate however - I think Rioja puts out over 10 times the amount of wine.
More importantly, Rioja has well over 1000 bodegas whereas Jumilla has about 45 and some of those are co-ops. Many bodegas in the region just don't produce wine that's all that good, and consequently it's not imported into the US.
There are a few producers in the region who worked with a few US importers to develop wines for the US market and those dominate what people see and know.
There are also newer and younger producers who are ambitious and working to upgrade the quality of the wine - most of them aren't here and the more ambitious are also hindered by the fact that their wines are costly and Spanish wine over $30 is just not moving in the US market, even with really high points.
Otherwise, it's still a region of rustic wines.
In the $10 - $20 range, I'm hard pressed to think of all that many wines from Jumilla. It's essentially a small group of wines that define Jumilla in the US - Julia Roch e Hijos Casa Castillo, Casa de la Ermita, Agapito Rico, Finca Luzón, Hijos de Juan Gil, and Bodegas Olivares, and Bodegas Bleda. Then there's Bodegas El Nido, which is about 9 years old and was started by a US importer who partnered with Juan Gil and brought in an Australian winemaker specifically to produce a confection for the US critics and market. Smart move too - he picked the winemaker with more 100 point wines from Parker than anyone else and they hit some home runs. But those wines are way over $20, and there are plenty of those in Rioja too.
I like many of the wines for sure and some of them are quite delicious, but IMHO there's no comparison to Rioja in terms of selection or availability.